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if; in case; supposing
もしも明日あす晴れはれたら出かけようでかけよう。
If it's sunny tomorrow, let's go out.
もしも時間じかんがあれば手伝ってつだって。
Help me if you have time.
もしも魔法まほうが使えつかえたらなあと子供こどもは夢見ゆめみた。
The child dreamed of being able to use magic.