also: てもよい
expression, i-adjective (yoi/ii class)
even if; even though; no matter how
Attaches to the -masu stem of a verb to express concession or compromise: 'even if one does...', 'no matter how...'. Often used with も in the main clause.
雨が降ってもいいように傘を持っていく。
I'll take an umbrella just in case it rains.
どんなに難しくてもいいから、挑戦してみたい。
No matter how difficult it is, I want to give it a try.
expression, i-adjective (yoi/ii class)
Attaches to the -masu stem of a verb to express permission or allowance: 'it's okay to do...', 'you may do...'. Often used in questions asking for permission.
ここで写真を撮ってもいいですか。
May I take a picture here?
もう帰ってもいいよ。
You can go home now.
ても構わない is a more formal or emphatic way to express permission or concession, literally 'it doesn't matter if...'. てもいい is more casual and common in everyday speech.
てはいけない expresses prohibition ('must not do'), the opposite of permission. てもいい expresses permission or concession.
Combination of the conjunctive particle て (from the -masu stem of a verb) + も (inclusive particle) + いい/よい (adjective meaning 'good'). Literally 'even if ... it is good', which developed into expressions of concession and permission.