てもいい

1. 1Asks whether an action is allowed ("May I ~?").Ex. 1
2. 2Grants permission or states that an action is acceptable ("You may ~ / It's fine to ~").Ex. 2

A neutral pattern used in everyday speech. It becomes polite with です, and よ softens it in casual speech. Compared with てもかまわない, it is simpler and more common. It is the opposite of てはいけない, which forbids an action.

• 〜てもいいですかN5「〜てもいいですか」is the polite question form of 「てもいい」 and attaches the same way, but it is used only to ask for permission or to check whether a condition is acceptable. 「てもいい」 can also state or grant permission, as in 「休んでもいいよ」.
• てもかまわない / てもかまいませんN3「てもかまわない」has essentially the same meaning and attachment as 「てもいい」, and the two are interchangeable in most cases. It sounds slightly more formal, with a 'don't mind' nuance, and 「てもかまいません」appears mostly in polite contexts such as customer service and announcements.
• てはいけないN5「てはいけない」means the opposite, expressing prohibition — that something 'won't do': to prohibit, say 「てはいけません」 rather than 「てもよくない」, and to refuse tactfully, say 「ちょっと……」. Both attach to the te-form but cannot be interchanged.
• なくてもいいN4「てもいい」gives permission to do something or accepts a certain state, while 「なくてもいい」attaches to the nai-form and means it is okay not to do something — an exemption from obligation — or that things are fine even without something or not being in a certain state. The two do not contradict each other and can hold at the same time, but they cannot be interchanged.
• ても差し支えないN1「ても差し支えない」has essentially the same meaning and attachment as 「てもいい」 but is the most formal in tone, used mainly in business, notices, and written language. To ask for permission formally, say 「ても差し支えないでしょうか」; in casual everyday conversation it sounds stiff.

〜てもいいですか is the polite question form of this pattern for asking permission. てもかまわない and ても差し支えない express the same permission with increasing formality, with ても差し支えない being stiff and formal; てはいけない forbids an action, and なくてもいい says an action is unnecessary.