もし

もし means "if". It goes at the front of a conditional clause and warns the listener that a supposition is coming. It does not make the condition by itself: the clause still ends in 〜～たら, 〜～ば or 〜～なら, exactly as it would without もし.

Drop もし and each sentence still means the same. With it, the "if" is clear from the first word, which helps most in a long sentence.

It needs a conditional ending

もし cannot hold up a clause that ends in a plain form, and it does not go with とき. The ending carries the condition; もし only announces it.

• × もし雨が降る、家にいます → もし雨が降ったら、家にいます
• × もし暇なとき、電話してください → もし暇なら、電話してください

"If", not "when"

On its own, ～たら also means "when" for something that is sure to happen. もし rules that reading out: it marks the condition as one that may not happen.

So もし does not go with something certain.

• × もし明日になったら、電話します → 明日になったら、電話します

Usage notes

These go beyond a plain "if".

Something that isn't true

With ～たら or ～ば, もし also sets up a situation contrary to fact. English switches to "were" or "had"; Japanese keeps the same ending.

もしよかったら

もしもし

The telephone greeting もしもし is a separate word. It comes from もうす申す "to say", not from this もし.