もしも

もしも is a stronger もし, "if" or "supposing". It stands at the head of a conditional clause and marks the situation as imagined – unlikely, a worst case, or not true at all. Like もし, it does not make the condition by itself: the clause still ends in 〜～たら, 〜～なら, 〜～ば or 〜～ても.

• × もしも雨が降る、試合は中止です → もしも雨が降ったら、試合は中止です

Only for what might not happen

On its own, ～たら also means "when" for something that is sure to happen. もしも leaves only "if", so it cannot go with something certain.

• × もしも明日になったら、電話します → 明日になったら、電話します
• × もしも夏休みが終わったら、国に帰ります → 夏休みが終わったら、国に帰ります

もしも～たら

This is the ending もしも meets most often. A verb takes its た-form plus ら, an い-adjective becomes 〜かったら, and a noun or な-adjective takes だったら.

• × もしも高いだったら → もしも高かったら

"What would you do if…?" is the question this ending is made for.

もしも～なら

With なら, "if that is the case": a supposed fact, a plan, a state. なら follows the plain form of a verb or an い-adjective, and a noun or な-adjective directly, with no だ.

• × もしも明日雨だなら → もしも明日雨なら

もしも～ば

A verb or an い-adjective goes into its ば-form: 〜えば for a verb, 〜ければ for an い-adjective. ば suits a state – ある, an adjective, a potential verb. When the first half is an action and the second half asks for something, use たら instead.

• × もしも安いば → もしも安ければ
• × もしも道に迷えば、電話してください → もしも道に迷ったら、電話してください

Even if: もしも～ても

With ～ても the condition becomes "even if": whatever happens, the second half still holds. A verb goes into the て-form plus も, an い-adjective into 〜くても, and a noun or な-adjective takes でも.

• × もしも失敗するでも → もしも失敗しても

もしも or もし

Both go with the same endings, and もし is the everyday one. もしも leans harder on the "suppose": a dream, a worst case, a thought experiment.

Usage notes

These go beyond a plain supposition.

Things that are not true

With たら or なら, もしも can set up a situation contrary to fact. English switches to "were" or "had"; Japanese keeps the same ending for the present, and a past one usually takes ～ていたら. The second half often ends in のに or だろう.

ならば

ならば is the full form of なら. It sounds more formal and turns up in writing and speeches.

もしものとき, もしものこと

もしも also works as a noun for an emergency or something bad happening: もしものとき "in an emergency", もしものことがあったら "if anything happens".