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Conditional Statement

How to say "if" in Japanese
This whole section is dedicated to learning how to say "if" in Japanese. Oh, if only it was as simple as English. In Japanese, there's four (count them, four) ways to say "if"! Thankfully, the conjugations are sparse and easy especially since you don't have to deal with tenses.

Expressing natural consequence using 「と」
We'll first cover the simplest type of "if" which is the natural consequence conditional. This means that if [X] happens, [Y] will happen as a natural consequence. No question about it. If I drop a ball, it will fall to the ground. If I turn off the lights at night, it will get dark. We can express this type of condition in the following format.

Rules for using the conditional 「と」
Attach 「と」 to the condition followed by the result that would occur should the condition be satisfied
= [Condition] + と + [Result]
State of being must be made explicit
= [State of Being] + だと + [Result]

Examples
（１）　ボールを落すと落ちる。
- If you drop the ball, it will fall.

（２）　電気を消すと暗くなる。
- If you turn off the lights, it will get dark.

These examples are designed to show how 「と」 is used to express natural consequence. However, even if the statement isn't a natural consequence in itself, the 「と」 will tell the audience that it is nevertheless expected to be a natural consequence.

（１）　学校に行かないと友達と会えないよ。
- If you don't go to school, you can't meet your friends.

（２）　たくさん食べると太るよ。
- If you eat a lot, you will get fat, for sure.

（３）　先生だと、きっと年上なんじゃないですか？
- If he's a teacher, he must be older for sure, right?

The "for sure" part is the implied meaning supplied by the 「と」. The speaker is saying that the following condition will occur in that situation, no matter what. As you can see from the last example, if the condition is a state-of-being, it must be expressed so explicitly using 「だ」. This applies to all non-conjugated nouns and na-adjectives as I'm sure you're used to by now. This will also help prevent confusion with other types of 「と」.

Contextual conditionals using 「なら（ば）」
Another relatively easy to understand type of "if" is the contextual conditional. You can use this particle to express what will happen given a certain context. For example, if you wanted to say, "Well, if everybody's going, I'm going too" you would use the 「なら」 conditional because you are saying that you will go in the context of everybody else going. The contextual conditional always requires a context in which the conditional occurs. For instance, you would use it for saying things like, "If that's what you are talking about..." or "If that's the case, then..."
In a sense, you are explaining what would occur if you assume a certain condition is satisfied. In other words, you are saying "if given a certain context, here is what will happen." You will see this reflected in the English translations as the phrase "if given" in the examples.

The 「なら」 is attached to the context in which the conditional occurs. The format is the same as the 「と」 conditional, however, you must not attach the declarative 「だ」.

Rules for using the contextual conditional 「なら」
Attach 「なら」 to the context in which the conditional would occur
= [Assumed Context] + なら + [Result]
You must not attach the declarative 「だ」.

Examples
（１）　みんなが行くなら私も行く。
- If given that everybody is going, then I'll go too.

（２）　アリスさんが言うなら問題ないよ。
- If given that Alice-san says so, there's no problem.

Example Dialogue アリス）　図書館はどこですか。
- Where is the library?

ボブ）　図書館なら、あそこです。
- If given that you're talking about the library, then it's over there.

The following is incorrect.
（誤）　図書館だならあそこです。

You can also decide to use 「ならば」 instead of just 「なら」. This means exactly the same thing except that it has a more formal nuance.

General conditionals using 「ば」
The next type of conditional just expresses a regular "if" condition without any assumptions or embedded meanings. However, it cannot be used with nouns and na-adjectives. Conjugation-wise, the 「ば」 conditional is fairly easy. For verbs, like the potential form, you change the last / u / vowel sound to an / e / vowel sound but instead of attaching 「る」, you attach 「ば」. This rule also applies to ru-verbs. For i-adjectives and negatives ending in 「ない」, you take off the last 「い」 and add 「ければ」.

Conjugation Rules for 「ば」
For verbs, just like the potential form, change the last / u / vowel sound to the equivalent / e / vowel sound and attach 「ば」
（例） 食べる　→　食べれ → 食べれば
（例） 待つ　→　待て → 待てば
For i-adjectives or negatives ending in 「ない」, drop the last 「い」 and attach 「ければ」.
（例） おかしい　→　おかしければ
（例） ない　→　なければ

Examples
（１）　友達に会えれば、買い物に行きます。
- If I can meet with my friend, we will go shopping.

（２）　お金があればいいね。
- If I had money, it would be good, huh?

（３）　楽しければ、私も行く。
- If it's fun, I'll go too.

（４）　楽しくなければ、私も行かない。
- If it's not fun, I'll also not go.

（５）　食べなければ病気になるよ。
- If you don't eat, you will become sick.