Conditional Form

Drop the final -u from the plain form of the verb and add -eba. To form the negative, drop the -i from the negative plain form and add -kereba. With -i adjectives, drop the final -i and add -kereba; with negatives, drop the final -i from nai and add -kereba.

もしできれば、今年外国へ行きたいんです。
Moshi dekireba, kotoshi gaikoku e ikitai n desu.
If I can, I want to go abroad this year.

辞書を使わなければ、この日本語の宿題ができません。
Jisho o tsukawanakereba, kono Nihongo no shukudai ga dekimasen.
If I don't use a dictionary, I can't do this Japanese homework.

明日天気がよければ、どこかへ行きましょうか。
Ashita tenki ga yokereba, dokoka e ikimashou ka.
If the weather's nice tomorrow, shall we go somewhere?

高くなければ、買います。
Takaku nakereba, kaimasu.
If it's not too expensive, I'll buy it.

Using conditionals to express obligation

The expression -nakereba narimasen, where naru is the verb "to become", means literally "if you don't..., it's no good" or in other words "you must" or "you have to". The negative "don't have to ..." is expressed with -nakute mo ii desu.

自己紹介は日本語でなければなりません。
Jiko shoukai wa Nihongo de nakereba narimasen.
Your self-introduction must be in Japanese.

今日終らなくてもいいです。
Kyou owaranakute mo ii desu.
You don't have to finish it today.

Past conditionals

もし時間があったら、あのテストは満点だったのに。

もしあの時ピザを食べなかったら、彼女にプレゼントを変えたのに。
Moshi ano toki piza o tabenakattara, kanojo ni purezento o kaeta noni.
If I hadn't eaten pizza (at) that time, I could have bought gifts for my girlfriend.