The term is infinitive, bare infinitve, to-infintive. I have never seen the term infinitival. And as to your verb constructions, it depends on what you want to say.

1 I heard the man singing in the street.

That is: The man was singing in the street. + I heard him.

2 I heard a man sing in the street.

That is: A man sang in the street. + I heard him.

You can put stress on the action in progress as in 1 or on the fact as in 2.

To help is a different case. The normal construction is to help to do/ to help someone to do (do stands for any infinitive you want to use. Nowadays to help is seen as something similar to a modal verb and "to" is often dropped.

The term is infinitive, bare infinitve, to-infintive. I have never seen the term infinitival. And as to your verb constructions, it depends on what you want to say.

1 I heard the man singing in the street.

That is: The man was singing in the street. + I heard him.

2 I heard a man sing in the street.

That is: A man sang in the street. + I heard him.

You can put stress on the action in progress as in 1 or on the fact as in 2.

To help is a different case. The normal construction is to help to do/ to help someone to do (do stands for any infinitive you want to use). Nowadays to help is seen as something similar to a modal verb and "to" is often dropped.