which one is correct ?

I normally use fill in a blank on a form. So, I would say All you need to do is complete the/this form.

You mean the verb 'be' is followed by plural verb, right?

tenjing You mean the verb 'be' is followed by plural verb, right?

Followed by the infinitive. The infinitive clause is the complement of the main verb "be."

All you need to do is ( to ) fill up the form.

So 'to' is optional, right??

I commonly say and hear fill in this form.
Clive

I've read somewhere that we could say: all you need to do is to do sth or do sth ( but doing sth is not grammatical)/ the writer also mentioned that the sentence with "to" is more formal and the one without it is the more normal.

One of the difficult parts of English is understanding model verbs. Here, you can't say or write just 'understand', it should be a gerund. I think so.

nazanin saryazdiI've read somewhere that we could say: all you need to do is to do sth or do sth ( but doing sth is not grammatical)

Correct. The -ing form is only used with "doing".

What you should be doing is studying for your history exam.

CJ

CJ, do you mean mine is incorrect?

tenjing One of the difficult parts of English is understanding modal verbs.
Here, you can't say or write just 'understand', it should be a gerund. I think so.

Correct. You have to have a gerund in the case shown by the underlined sentence. There's no "what" or "do" in the first part.

Note the differenc