てもいいMay or can (permission)
てもいい says that someone is allowed to do an action. In a question, it asks for permission.
Giving and asking for permission
A statement with てもいい grants permission. Adding the question ending asks whether the action is allowed.
A basic permission exchange
この資料をコピーしてもいいですか。
May I copy this document?
Analyzeはい、コピーしてもいいですよ。
Yes, you may make a copy.
AnalyzeThe first turn requests permission; the second grants it.
When the te-form ends in the voiced form
休憩中はここで飲んでもいいです。
You may drink here during the break.
Analyze飲む has the te-form 飲んで, so the result is 飲んでもいい.
Doing an action versus not doing it
てもいい permits the action itself. By contrast, なくてもいい says that not doing the action is acceptable, so the action is optional.
明日は来なくてもいいです。
You do not have to come tomorrow.
AnalyzeLiterally: It is fine even if you do not come tomorrow.
More examples
窓を開けてもいいですか。
May I open the window?
Analyzeここに座ってもいいですよ。
You may sit here.
Analyzeこの辞書を使ってもいいですか。
May I use this dictionary?
Analyze授業が終わったら、帰ってもいいです。
You may leave when class is over.
Analyze宿題が終わったら、遊んでもいいよ。
You may play after you finish your homework.
Analyzeここで写真を撮ってもいいですか。
May I take pictures here?
Analyze