The word for "fall": taoreru vs. korobu (倒れる vs. 転ぶ)

倒れる "taoreru" is used for objects that fall down while stationary. Therefore, this is used for inanimate objects and for people who collapse.

転ぶ "korobu", on the other hand, is used for objects that fall while moving, so this is used for events like slipping while skiing or skating.

Reference:
Akira Miura, Essential Japanese Vocabulary, Tuttle

Different kanji for "akeru" あける：開ける、空ける、明ける

Jim Breen's EDICT doesn't really provide the difference between the three different ways to write "akeru": 開ける、空ける、明ける, so I'm going to write this article for my reference. I hope others will find this useful as well. All definitions come from Kenkyusha's 5th Shinwaei Chuujiten and the Daijiten (研究者新和英中辞典・大辞典) as well.

"How long" in Japanese

The question phrase "how long" is usually expressed using by any of the following: どのくらい and どれくらい. This source says that, according to the 日本文法大辞典, どのくらい was commonly used in the Edo era, but どれくらい became more common in the Meiji era. Google Search shows that both are still commonly used today. くらい is loosely translated as "about" or "approximately".

どのくらい/どれくらい待ちますか。
Dono  kurai/dore kurai machimasu ka?
How long will we wait?

 あなたは日本にどれくらい住んでいますか？
Anata wa Nihon ni dorekurai sunde imasu ka?
How long have you been living in Japan?

You can also be more specific by using 何日、何時間、何ヶ月, and so on.

毎日、何時間、仕事をしますか。
Mainichi, nanjikan, shigoto wo shimasu ka?
How many hours do you work every day?

何日ぐらい滞在しますか。
Nannichi gurai taizai shimasu ka?
About how many days will you stay?


Source:
jisho.org
Google search
Japanese for everyone

「そのとなり」

More commonly, we encounter position words such as となり, そば, まわり etc in the following grammatical construct:

____ の<となり、まわり, etc>

Interestingly, one of the things that is not taught is the construction

この・その・あの<となり、まわり, etc>

I've seen only そのとなり used (Japanese for Everyone by Nagara), and it means "the one beside it". However, I see no reason why it cannot be used for この・あの. Apparently, it's not very commonly used with そば, although it is commonly used with まわり based on Google Search.

The word for "have": 持っている vs. には～がある/いる

Disclaimer: I didn't get this from any book, but this is something that I picked up by hearing these two expressions many times.

Consider the following sentences in English:

1. I have a car.
2. I have two brothers.
3. I have talent.

In English, the idea of possession of a "car", "brother", or "talent" are all handled by the same word, "have". However, in Japanese, there's another nuance that has to be taken care of.

What's the difference between having a car and a brother? One difference here is that you will always have a brother - it's something that's part of you - while you can buy or discard a car. Having a "brother" is inherent, and having a "car" is not. Note that this divide also holds for possession of animate/inanimate objects. Having a "cat" is inherent, but having a "house" is not.

In Japanese, these two nuances are covered by two different sentence patterns: ～を持っている (lit. to hold), and ～に～がいる/ある (lit. to exist in).

The word for convenient: べんり、つごうがいい

Elevators, escalators, bullet trains are べんり "benri" . They make life easier, and in that sense, are "convenient". Having to ride the train during crowded rush hour is ふべん ("fuben") "inconvenient"。

On the other hand, when something is convenient in the sense that it matches your schedule, the word to use in not べんり, but rather つごうがいい. When something is inconvenient for your schedule, then you use つごうがわるい。

How to say "popular" in Japanese: はやる and 人気

はやる "hayaru" and 人気 "ninki" are two different words that are both translated as "popular" in English. However, they are not exactly the same.

というか and ということ "to iu ka" and "to iu koto"

Much of this post was directly from Tae Kim's Guide to Japanese Grammar since I'm too lazy right now to write my own explanations.

直す vs. 治す "naosu"

There are two kanji for "naosu": 直す and  治す. They have different uses:

 治す = cure (a disease, a person)
直す = repair (something broken)

How to use まるで

まるで "marude" has two basic meanings:

1. "just like"

When used in this manner, まるで is often followed by よう or みたい.

スミスさんは日本語が上手で、まるで日本人（のよう）だ。
Sumisu san wa Nihongo ga jouzu de, marude Nihonjin no you da.
Smith is good in Japanese that he speaks like a Japanese.

あの老人はまるで赤ん坊（みたい）だ。
Ano roujin wa marude akanbou mitai da.
That old man is just like a baby.

2. "completely"

In negative sentences, まるで has the same function as まったく "mattaku" and ぜんぜん "zenzen". However, まるで implies the speaker's negative judgement.

Reference:
Miura, Akira. Essential Japanese Vocabulary, Tuttle.

Southeast or east-south?

In Japanese, there are two ways to express southeast: 東南 "tounan" = "east-south" or 南東 "nantou" = "south-east". However, they have different uses. Use "east-south" if you're talking about a particular location, e.g., Southeast Asia = 東南アジア. When talking about a particular direction, use "south-east": southeasterly wind = 南東の風.

Reference:
Miura, Akira. Essential Japanese Vocabulary, Tuttle Publishing.

The word for "help":「助ける」と「手伝う」 tasukeru vs. tetsudau

Both words mean "help" in English. However,　手伝う "tetsudau" refers to helping someone with the same task. 助ける "tasukeru" can be used in place of 手伝う in some cases. However, unlike 手伝う, it can be used to refer to saving or rescuing someone (for this, 手伝う is NEVER used!) . From my encounters with the language, 助ける is more often used in this context of rescue than just helping someone complete a chore.

The word for "sun":「太陽」と「日」 taiyou vs. hi

Both 太陽 "taiyou" and 日"hi" mean "sun" in Japanese. However, 日 is a more anthropocentric term, meaning it refers to the sun as something in relation to human activities. If you're talking about the sun setting or rising, then 日 is the proper word to be used.

太陽, on the other hand, is used to refer to the sun as a body in the solar system. It is more scientific than 日. Thus, we have the words

太陽の黒点
taiyou no kokuten
sunspots

太陽の直径
taiyou no chokkei
diameter of the sun

I've also seen 太陽 used in songs:


恋してるチカラに魔法をかけて, 太陽がずっとしずまないように
koishiteru chikara ni mahou wo kakete, taiyou ga zutto shizumanai you ni
Put a spell on loving power so that the sun wouldn't set.

ほら、太陽が優しい風が、ぼくらを見つめているから
Hora taiyou ga, yasashii kaze ga, bokura wo mitsumete iru kara
Look, the sun and the gentle wind are gazing at us.

In these two examples, the sun seems to be considered as an animate object, almost like personification.

References:
Akira Miura, Essential Japanese Vocabulary, Tuttle Publishing.

「このごろ」と「このあいだ」 kono goro vs. kono aida

「このごろ」 "kono goro" means "these days", while 「このあいだ」"kono aida" means "the other day". That is, "kono goro" refers to something which has been going on for a while, while "kono aida" is used for a particular event that occurred recently.

The word for "world": "sekai" and "yononaka"「世界」と「世の中」の違い

The Japanese have many words for the word "world". However, they are not exactly interchangeable.

世界 "sekai" is commonly used for the physical world composed of the different countries and continents. In terms of nuance, it is related to 地球 "chikyuu", which literally means "the earth". Basically, 世界 can exist even without us human beings. Here are some real-life examples that come from some documentaries I've watched or from jisho.org.

世界選手権
sekai senshuken
World Championships

世界の頂点
sekai no chouten
top of the world

世界には7大陸がある。
Sekai niwa nanatairiku ga aru.
There are 7 continents in the world.

愛の力は世界を動かす。
Ai no chikara wa sekai wo ugokasu.
Love makes the world go round.

世の中 "yononaka", on the other hand, is used to refer to society instead of the "physical" world that 世界 is often used for. It is related to words like 世間 "seken" and 社会 "shakai", which both mean "society". (More on those two words later."

世の中は狭いものですね。
Yononaka wa semai mono desu ne.
It's a small world, isn't it?

彼は世の中を正気に戻した。
Kare wa yononaka wo shouki ni modoshita.
He brought the world back to its senses.

It seems though that there is some overlap, based on example sentences:

僕は新世界の神となる。
Boku wa shinsekai no kami to naru.
I will become a god of the new world. (from Death Note)


References:"Essential Japanese Vocabulary" by Akira Miura
jisho.org (for the example sentences)
http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1088088560
http://okwave.jp/qa/q4404083.html

The word for "improve": "joutatsu" and "koujou"「上達」と「向上」

Eijiro gives the following words for "improve": joutatsu suru 「上達する」and koujou suru 「向上する」. However, they are not identical and I've done some research on the difference.

Let's start with the sample sentences from jisho.org (which uses the Tanaka corpus):

君の英語は上達している。
Kimi no Eigo wa joutatsu shite iru.
Your English is improving.

彼の英語力は著しく向上した。
Kare no Eigoryoku wa ichijirushiku koujou shita.
His English abilities have remarkably improved.

Note that joutatsu was used with Eigo, while koujou was used with Eigoryoku. This gives us one difference between the two: for 上達, what is relevant is the thing which is being improved on (in this case, English), while for 向上, it refers to something that goes "up" -- in this case, it is the English ability, and not English the language per se. If we say

英語は著しく向上した。
Eigo wa ichijirushiku koujou shita.
English (the language) has remarkably improved.

then we get a completely different meaning.

In some cases (like English ability), 上達 and 向上 can both be used with a minor change in sentence construction. However, in some cases, only one of the verbs can be used. 上達 is used for things where you can improve with practice, like sports, language abilities, piano skills, etc. 向上 has a wider scope, and includes improvements in condition, living standards, production, etc.

Here's a thread (in Japanese) about the same question. One of the replies says that 上達 means to "get better at something", while 向上 is more like "to advance in a good direction".

Reading practice: children's newspapers

Fortunately, some Japanese newspapers do release children's versions, such as the Yomiuri Shimbun and NHK News. This is great for practicing your kanji and increasing your vocabulary.

1. NHK News Web Easy
http://www3.nhk.or.jp/news/easy/

This one has furigana all over. Some people don't like it, but I do. I can read the kanji that I know without looking at the furigana, and then I just check the furigana after I finish guessing the reading. Also, the articles here are more recent. Best of all, this has AUDIO, so you also get to practice your listening skills as well!

2. Yomiuri Shimbun
http://www.yomiuri.co.jp/nie/note/kids/index.html

The nice thing about the Yomiuri kids' edition is that you can find articles by grade level. So if you're only confident about your kanji level being 3rd grade, then you can still read articles that don't require higher level kanji. Kanji that at are above your grade level are spelled out in furigana.

朝、午後、午前中 asa, gogo, gozenchuu

午後 ("gogo") means both "afternoon" and "P.M":

あしたの午後来てください。
Ashita no gogo kite kudasai.
Please come tomorrow afternoon.

いまは午後２時です。
Ima wa gogo niji desu.
It's now 2PM.

午前 ("gozen") means "A.M.":

いまは午前２時です。
Ima wa gozen niji desu.
It's now 2AM.


朝 ("asa") means "morning", but only until mid-morning (around 9 or 10AM). It is not used in the sense of "A.M."


あしたの朝来てください。
Ashita no asa kite kudasai.
Please come tomorrow morning (before mid-morning!).

If you wish to tell someone that he can come before noon, you can use 午前中("gozenchuu ") :
あしたの午前中来てください。
Ashita no gozenchuu kite kudasai.
Please come tomorrow morning (anytime before noon).

Or you can also write
あしたの正午前に来てください。
Please come tomorrow morning before noon.


Main reference: Essential Japanese Vocabulary by Akira Miura, Tuttle Publishing.

「必ず」と「きっと」 kanarazu vs. kitto

必ず (kanarazu) means "without fail", while きっと means "with high probability". For sentences which mean that something is or should be at or near 100% probability, we use 必ず:

人間はだれでも必ず（ｘきっと）しぬ。
Ningen wa daredemo kanarazu (not "kitto") shinu.
All human beings will die without exception.

明日の朝までに必ず宿題をだしてください。
Ashita no asa made ni kanarazu shukudai wo dashite kudasai.
Make sure to give your homework by tomorrow.

きっと (kitto), on the other hand, refers to the speaker's personal beliefs or inference.

田中さんはきっと来るよ。
Tanaka-san wa kitto kuru yo.
I'm sure Tanaka-san will come.

For past events, one can use きっと when talking about a personal evaluation of what happened:

太郎はきのうクラスを休んだから、きっと（ｘ必ず）病気だったのだろう。
Tarou wa kinou kurasu wo yasundakara, kitto (not "kanarazu") byouki datta no darou.
Because Tarou was absent yesterday, I'm pretty sure he was sick.

But when one is talking about a past event that is verified to be accurate, one must use 必ず:

田中さんは、パーティーがあると必ず（ｘきっと）来た。
Tanaka-san wa, paatii ga aru to kanarazu (not "kitto") kita.

Note that the past sentence is not a personal evaluation.

Note also that 必ず is not normally used for negative sentences, but きっと may be so used.

出る vs. 出かける deru vs. dekakeru

出かける means to go out of one's home.
出る means just to go out of any place.

Further, 出かける indicates an outing of some sort -- going to class, going to the mall, etc. If you're literally going just outside your house, 出る is more appropriate.