Question

pick out infinitive in sentence : i just had to do it ​

Answer 1

The infinitive in the given sentence is underlined:

I just had to do it.

Explanation

Infinitive is classified under the category of 'Non- finite' verbs: 1. Infinitives 2. Gerunds 3. Participles.

Non-finite verbs are those verbs that remain unchanged in any type of tense. Their form remains the same. They do not inflect.  

Infinitive = to + V1

Gerund = V1 + -ing (working as noun)

Participle = V1 + -ing (working as verb); V3 (working as adjective)

Answer 2

to do is the infinitive in the sentence.

When an infinitive is added to the subordinate clause then the subordinate clause becomes the main clause and can be understood on its own.

An infinitive clause typically contains to + verb in 1st form and in this sentence, it is to + do.

The other forms of do are doing and does