Conditionals: ～と, ～たら, ~ば,～なら

Lifted entirely from Derek Schaab's answer. Basically the idea is:

～と: when something is a natural result of some action. Hence, this result cannot be volitional.

～ば: the emphasis here is on the condition (whether it will be met or not is unsure), not the result

～たら: this is the most versatile conditional. It is used when the speaker is more or less sure that the condition will be met, and therefore the emphasis here is on the result. For example, translating "When you're done cooking, call me upstairs" would require ~たら, not ~ば, although there are cases where ~たら can be used instead of ～と or ~ば.

～なら: Probably the most interesting conditional in terms of nuances. Schaab provides a good explanation. It is unique among all conditionals in that it allows you to give a "result" before the condition is met.

Ex. もううちに帰るのなら、この手紙をお母さんに上げてくれませんか。If you're already going home, could you please give this letter to your mom?

Note that the speaker here is requesting an answer BEFORE the speaker goes home.

As we can see, ～なら gives a "while you're at it" or "if you're going to do it anyway" feel. Given that the conditional clause is met, what conclusion or request can you draw from it? That's what ～なら does. This is why, in general, it is not interchangeable with ～と, ～たら or ~ば, all of which require that the condition is met first.

Anyway, let's see Schaab's original answer, lifted verbatim:

~と

• と, ば: The main clause must be a constant non-volitional reaction to the conditional clause unless the conditional clause shows state or if the subjects of the two clauses differ.

お金を入れてボタンを押すと、切符が出ます。
'When you put in money and press the button, a ticket will come out.'
春になると、観光客が増えます。
'When spring arrives, tourists increase.'
早く行かないと、遅刻します。
'If you don't leave soon, you'll be late.'
× お家に帰ると、連絡してください。
お家に帰ったら、連絡してください。
'Please contact me once you return home.'
× パリに行くと、凱旋門がいせんもんにも行ってみたい。
パリに行ったら、凱旋門にも行ってみたい
'When I go to Paris, I'd like to see the Arc de '.
× 帰宅すれば、必ずお風呂に入りなさい。
帰宅したら、必ずお風呂に入りなさい。
'When you get home, be absolutely sure to take a bath.'
わからないことがあったら、いつでも聞いてください。
わからないことがあれば、いつでも聞いてください。
'If there's something you don't understand, ask me anytime.'
父が許してくれれば、彼と結婚するつもりです。
'If my father lets me, I intend to marry him.'

• と: The conditional clause must be non-past.

× 窓を開けたと、冷たい風が入ってきた。
窓を開けると、冷たい風が入ってきた。
'When I opened the window, a cool breeze came in.'
× デパートに行くと、チョコレートが山積みになっていた。
デパートに行くと、チョコレートが山積みになっていた。
'When I went to the department store, I found mountains of chocolate piled up.'

～ば

• The condition must have not happened yet.
  明日もし雨が降れば、どうしますか。
  明日もし雨が降ったら、どうしますか。
  × 明日もし雨が降ると、どうしますか。
  'If it rains tomorrow, what should we do?'
  × 注射を打ってもらえば、すぐ直りました。
  注射を打ってもらったら、すぐ直りました。
  'When I had the injection, I got better right away.'
  
  
  ～ば is the most common conditional when the emphasis is on what is required to bring about a desired result. Thus, use ～ば when the focus of the sentence is on the conditional clause:
  
  
  どうすれば、東大に入学できますか。 What should I do to get into Tokyo University?
  
  
  Consequently, when the succeeding clause is negative, ～ば sounds unnatural:
  
  
  ？ 徹夜すれば、体調が悪くなります。
  Instead: 徹夜すると、体調が悪くなります。 If you stay up all night, you'll damage your health. (～たら is also OK)
  
  
  But when the preceding clause uses さえ to show the minimal criteria needed to achieve a result, ～ば is your only option:
  
  
  お金さえ払えば、だれでも入会できる。 So long as they pay, anyone can join. (Neither ～たら nor ～と work here.)
  
  
  Use when talking about what would happen if something (which is not actually true) were true:
  
  
  あと1000円あれば、このコートを買えるのに。 If I had 1,000 yen more, I could buy this coat.
  あのとき右に曲がれば、どうなっただろう。 If I had turned right back then, I wonder what would have happened.
  
  
  Note that the tense of the following clause shows whether you're speculating on the past or non-past, and that past-tense verbs are allowed in this case.
  
  
  
  
  

  ～たら

  1. Use when expressing a one-off (as opposed to constant or general) dependency. Like ～ば, this can be used when it is unknown whether the preceding clause will come true:

     雨が降ったら、試合は中止です。 If it rains, the game will be called off. (～ば is also OK)

     Unlike ～ば, ～たら can be used when it is a known fact that the preceding clause will come true:

     午後になったら、散歩に行きましょう。 In the afternoon ("When it becomes the afternoon"), let's go for a walk. (～ば is not OK)

  2. Use when the following clause shows intent, desire, or is a command/request:

     食事ができたら、呼んでください。 When the food's ready, call me. (～ば is not OK unless the sentence is one of the exceptional cases mentioned above)

  3. Use like ～と to show a sequential, cause-and-effect relationship between the two clauses:

     田中さんにメールを送ったら、すぐ返事が来ました。 When I texted Ms. Tanaka, I got a reply right away. (～と is also OK, but not ～ば)

  4. ～たら can also take the polite form (along with ～と), but this is usually heard only in formal settings:

     ご感想がございましたら、ぜひお寄せ下さい。 If you have any feedback, please send it to us.

  ～なら

  1. Use when you are drawing out a conclusion based on the first clause:

     A: スーパーに行ってくるよ。 I'm going to the supermarket.

     B: スーパーに行くのなら、しょうゆを買ってきて。 If you're going to the supermarket, bring back some soy sauce.

     大学院に進むなら、この本を読みなさい。 If you're going to go to graduate school, read this book.

  2. Use when you need the freedom to have the succeeding clause happen before the preceding clause. This is something ～と, ～ば, and ～たら cannot do:

     旅行に行ったのなら、写真を見せてください。 If you went on vacation, please show me your photos. (A→B)

     旅行に行くのなら、カメラを持っていくといいですよ。 If you're going on vacation, you should take a camera. (B→A)

     飲んだら、乗るな。乗るなら、飲むな。 If you've had a drink, don't drive. (A→B) If you're going to drive, don't drink. (B→A)

     Do note that the rule of drawing out a conclusion based on the first clause still applies in these examples.

  Alternate forms:

  • なら will sometimes have a の or ん in front of it; the meaning does not change.
  • In speech, のなら sometimes becomes のだったら (or んだったら), again with no change in meaning.
  • I believe ならば is a more formal, written version of なら, but I don't have a solid reference on this.

  ～としたら

  This is one of many forms of ～とする. The ～とする form is used when setting up a supposition as the basis for succeeding statements:

  有名人にばったり会うとしたら、どうする？ Supposing you happen to meet someone famous, what would you do?

  新しい車を買ったとしても、それで幸せにはならないと思う。 Even supposing I were to buy a new car, I don't think that would make me happy.

  じゃあ、犯人はこの部屋に入ったとしよう。目的はなんだったんだろうね。 So let's suppose the criminal entered this room. What do you imagine he was after?

  ～ことになったら

  This is a combination of the ～ことになる form ("end up", "turn out to") and the ～たら conditional:

  結婚することになったら、結婚式で歌っていただけませんか。 If it so happens that I get married, would you please sing for me at the wedding?

  海外に行くことになったら、手紙を毎週書いて送るよ。 If I end up going overseas, I'll write you every week.

  Variances among dialects

  As if this wasn't complicated enough, a study done in 1989 by Shinji Sanada showed that Tokyo speakers and Osaka speakers sometimes differ greatly in their use of conditionals. A few examples:

  もっと早く｛起きると／起きれば／起きたら｝よかった。 I should have woken up earlier.

  • Tokyo: 4% と; 94% ば; 2% たら
  • Osaka: 0% と; 20% ば; 78% たら

  右に｛行くと／行けば／行ったら｝、ポストが見えます。 If you turn right, you can see a mailbox.

  • Tokyo: 75% と; 16% ば; 8% たら
  • Osaka: 4% と; 13% ば; 83% たら