としたら / とすれば

1. 1Supposing a hypothetical situation occurs, states the likely result.Ex. 1
2. 2Assuming a premise is true, draws an inference or raises a question based on it.Ex. 2

Used when the speaker consciously frames something as an assumption and then reasons from it. This makes it common in speculation, planning, and discussion. Compared with plain conditionals like たら or ば, it stresses the act of supposing rather than a general or sequential condition. The two forms are largely interchangeable.

• ならN4When 「なら」 follows a noun or な-adjective, no 「だ」 is added (「雨なら」); besides making hypothetical conjectures, it often takes up a topic from the other party to offer a suggestion, as in 「勉強するなら毎日練習したほうがいい」. When simply making a hypothetical conjecture, the two are mostly interchangeable.
• たらN4「たら」 can also indicate doing the second action after the first is completed, or an unexpected discovery made after the first action; these two uses cannot be replaced with 「としたら」. When both express a hypothesis, 「としたら」 more strongly emphasizes the tentative 'suppose that...' assumption, and its second clause is usually a conjecture, question, or judgment.
• ばN4「ば」 can express conditions and general rules, and it forms 「〜ばいい／〜ばよかった」, the latter of which cannot be replaced with 「としたら」; when making a hypothesis or drawing an inevitable conclusion, the two are similar, but 「ば」 does not particularly emphasize the tentative 'suppose that...' assumption. 「ば」 requires inflection.
• にしたらN2「にしたら」 is not a hypothesis but 'viewed from ...'s standpoint'; it attaches directly to nouns denoting people or organizations, with no 「だ」. For example, 「親にしたら」 means 'from the parents' point of view', while 「親だとしたら」 means 'if they are the parents', so the two cannot be swapped.
• としてもN2「としても」 is a contrastive hypothetical conditional: even if the hypothesis holds, the second clause stays unchanged, whereas 「としたら／とすれば」 is a non-contrastive hypothetical conditional whose second clause is a conjecture, judgment, etc. based on it; their meanings are opposite, so they cannot be swapped. After a noun, 「だ」 before 「としても」 is sometimes omitted.
• としたって / としたところでN1「としたって」 is a contrastive concessive meaning 'even assuming that..., still'; the following clause is usually a negative judgment that the outcome stays the same or is useless, and it can also attach to personal nouns to express a standpoint. 「としたら」 is a non-contrastive hypothetical whose second clause is a conjecture, question, or inference, so the two cannot be swapped.

Unlike なら, たら, and ば, which are general conditionals, としたら / とすれば explicitly frame the condition as a supposition, while にしたら means 'from X's perspective'. としても and としたって / としたところで are concessive ('even supposing…'), and としたって / としたところで imply that the outcome won't change.