～てもいい

The structure ～てもいい is used to express or ask for permission. It conveys that something is allowed or acceptable – similar to "may," "it's okay to," or "you can" in English.

Giving permission

When used in a statement, ～てもいい indicates that an action is permitted.

Asking for permission

When used with か (or rising intonation in casual speech), ～てもいい becomes a polite way to ask whether something is allowed.

A common response to grant permission is どうぞ ("go ahead") or いいですよ ("sure"). To deny permission, ～てはいけない or ～てはだめ ("you must not") is used.

In practice a flat refusal is often avoided. Trailing off is the usual polite way out:

Nuance

～てもいい literally breaks down as the て-form + も ("even") + いい ("good"), so the underlying meaning is "even if you do ~, it's fine." This explains why it naturally conveys permission. In casual speech, いい can be replaced with だいじょうぶ大丈夫 for a softer tone: