GRAMMAR DETAIL

としたら

if · supposing that

Assumes a situation whose reality is undetermined and continues with a judgment or wish based on it.

Examples

もし一日休めるとしたら、海へ行きたい。

If I could take one day off, I would like to go to the ocean.

もし明日休みだとしたら、何をしたいですか。

If tomorrow were a day off, what would you like to do?

Structure: としたら follows a plain-form statement and treats it as a hypothetical situation for a judgment, wish, or question.
Basic usage: もし明日休みだとしたら、何をしたいですか。 asks about a wish under the assumption that tomorrow is a day off.

と packages a statement as a premise
- 休みだとしたら and 静かだとしたら retain present affirmative だ after a noun or na-adjective. 休むとしたら connects directly to a dictionary-form verb, so the form depends on the preceding word class.