How to say "X if Y"?

Question

I want to say Please tell me if you want to go tomorrow!.

I can translate Please tell me to ぼくにおしえてください and you want to go tomorrow to 明日あなたは行きたい,

but how do I connected both with a conditional if?

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I read this question and I don't think it's a duplicate, as that doesn't answer mine.

Answer 1

There are quite a few ways to express "X if Y." in Japanese.

Here are the more common ones roughly in the order of formality:

「もし/もしも + Y + なら/ならば + X」

「もし/もしも + Y + であれば + X」

「Y + なら/ならば + X」

「Y + であれば + X」

「（もし） + Y + だったら + X」

Thus, the sentence "Please tell me if you want to go tomorrow!." can be said in many ways. How you would want to phrase it would mainly depend, of all things, on the relationship between the speaker and listener.

In the order of formality, one might say:

「もし（も）[明日]{あす}[行]{い}かれたいのであれば、お[知]{し}らせください。」 Used honorifics. Probably not for beginning or intermediate learners.

「もし明日行きたければ（or 行きたかったら）、知らせてください。」 Neutral in every way. This is how many J-learners speak in my experience. On this level of formality, 「明日」 could be pronounced either あした or あす. Will sound natural either way.

「[明日]{あした}行きたいのなら、知らせてね。」 Pretty informal. Notice the change in how 「明日」 is pronounced in informal speech.

「[明日]{あした}行きたかったら、知らせてね。」　One step more informal.

I took the liberty of using 知らせる instead of おしえる as the former would sound more natural.

Answer 2

You can use several forms

A なら B : "in the event of A, B"

行きたいなら教えてください

Verb-past たら B: "when A, B"

行きたかったら教えてください

There are subtle differences between the two. Mainly, なら specifies an "if this is true" kinda tone, while たら can also be used for "when" like "when I got into the tub I started sweating" お風呂に入ったら汗が出始めた