てもいいmay; it's fine to
てもいい says it is fine to do something. As a question it asks permission: "may I?". As a statement it gives it.
How to form it
Verb in the て-form + もいい
開けて → 開けてもいい, 飲んで → 飲んでもいい. Polite いいです; to ask, いいですか. In conversation the も often drops: 開けていい？
With a noun or an adjective
Noun + でもいい ／ い-adjective 〜くてもいい
"Would be fine too": 鉛筆でもいい, 狭くてもいい. A な-adjective works like a noun.
Asking: てもいいですか
You ask before doing something that affects other people, or when you don't know the rule.
May I take pictures here?
Excuse me, may I go to the bathroom?
May I sit here?
よろしい is a more careful いい, for a customer, a boss or a stranger.
Answering
A yes is short: どうぞ, which is right to anyone, or いいですよ.
May I copy this document?
Yes, go ahead.
May I use this dictionary?
Yes, that's fine.
Between friends: うん、いいよ.
A no is rarely a flat いいえ. People say sorry and give a reason, or just trail off after ちょっと.
May I smoke here?
Sorry, here is a bit…
B never finishes. Trailing off is the no.
いいえ、〜てはいけません, or a flat だめです, is a no from someone in charge: a teacher or a guard.
Giving permission: てもいいです
It comes from whoever decides, or from someone passing on a rule: a teacher, a parent, a host.
You may leave when class is over.
You may play after you finish your homework.
So to a teacher or a boss it sounds as if you were the one in charge. To offer them something, people say どうぞ.
to your teacher
Sounds odd: この
You may use this umbrella.
To offer it: どうぞ、使ってください.
May, must not, don't have to
Three forms that go together. Word for word, they say:
You may use a dictionary.
"Even if you use one, it's fine."
You must not use a dictionary.
"If you use one, it's no good."
You don't have to use a dictionary.
"Even if you don't use one, it's fine."
も comes before いい, and は before いけない.
In conversation: ていい
Can I eat this?
Can I ask you something?
After a noun or an adjective
Then it says that this would do too.
I don't have a ballpoint pen. Is a pencil all right?
I don't mind if the room is small.
Any drink is fine.
Close neighbours
なくてもいいN5
Takes a duty away. As a question, it checks whether you have to.
Is it okay not to copy this document?
A request that looks like "may I?". もらって is in it, so the other person does it: "could you…?".
Could you open the window?
Check yourself
1"You may go home now."
もう
Choose an answer to see why.
2Your boss is looking for a pen, and you have one. Which is natural?
Choose an answer to see why.
3A colleague asks this, and you would rather she didn't. Which answer is friendly?
エアコンを
Choose an answer to see why.
More detail
About yourself
Said about your own action, it is an offer: you are willing, though not keen.
I can go, if you like.
More sentences
May I come in?
You may work here.
You may sit in this empty seat.