May I? ~てもいいですか?

Asking permission and Being polite go hand in hand. Here is how you do both in Japanese.

Construction: 
arrow Take the て form of a verb
arrow Add もいいですか?

電話を使ってもいいですか?
denwa o tsukattemo ii desu ka?
May I
use the phone?

休んでもいいですか?
yasundemo ii desu ka?
May I
take a break? (from work, from studying, etc)