～としたら・～とすれば・～とすると
Assume something is true and consider the result
You’ll be able to set up a hypothetical assumption before reasoning about what would follow from it.
もし一か月自由に旅行できるとしたら、北海道を一周したいです。
moshi ikkagetsu jiyuu ni ryokou dekiru to shitara, hokkaidou o isshuu shitai desu.
What do you think this sentence means?
Supposing we could travel freely for a month, I would want to travel all the way around Hokkaido.
Set up a hypothetical assumption
～としたら (or とすれば, とすると) sets up a purely hypothetical assumption or conditional scenario ("supposing that...", "if we assume that..."), followed by a consequence, question, or intention.
もし魔法が使えるとしたら、空を飛んでみたいです。
moshi mahou ga tsukaeru to shitara, sora o tonde mitai desu.
Supposing I could use magic, I would like to try flying.
彼の話が本当だとすれば、大変なことです。
kare no hanashi ga hontou da to sureba, taihen na koto desu.
If what he says is true, it is a serious matter.
Attach としたら to a plain clause
Attach としたら, とすれば, or とすると directly to the plain form of verbs, adjectives, or nouns (noun + だ/である). It often pairs with the adverb もし.
引っ越すとしたら、静かな郊外に住みたいです。
hikkosu to shitara, shizuka na kougai ni sumitai desu.
Supposing that I were to move, I would want to live in a quiet suburb.
もし生まれ変われるとしたら、猫になって一日中寝ていたい。
moshi umarekawareru to shitara, neko ni natte ichinichijuu nete itai.
If I could be born again, I’d like to be a cat and sleep all day.
駅まで歩いて二十分かかるとすると、八時には家を出なければならない。
eki made aruite nijippun kakaru to suru to, hachiji ni wa ie o denakereba naranai.
If it takes twenty minutes to walk to the station, then I have to leave home by eight.
会議が中止になったとすれば、誰かが連絡してくれるはずだ。
kaigi ga chuushi ni natta to sureba, dareka ga renraku shite kureru hazu da.
If the meeting has been canceled, someone should let us know.
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