All I can do is say thank you.

Hi, Vegnlove,

@vegnlove posted:

This sentence below:

"All I can do is say thank you"

Q. How can verb 'say' come after 'is'?

Shouldn't it be 'to say' or 'saying'?

That sentence is correct. The sentence is called pseudo-cleft, and if you use "do," "did" or "done" in the subject clause of this kind of sentences, you need to use a bare infinitive (or a "to"-infinitive) in the complement clause. "Saying" would only be possible if you had "doing" in the subject:

- All I do (in these cases) is (to) say thank you.
- All I did was (to) say thank you.
- All I have done is (to) say thank you.
- All I am doing (now) is saying thank you.

Thank you very much!