The bare infinitive is used:

After modal verbs (can, should, must, etc.).

But: Ought and have are followed by to –infinitive.

We have to dothis experiment.

After the verbs let, make, see, hear and feel.

But: be made, be heard, be seen + to –infinitive (passive).

They saw her talkto the scientist.

She was seen to talkto the scientist.

After had better, would rather and would sooner.

You had better finishyour scientific project.

Help is followed by either the to –infinitive or the bare infinitive.

She helped me (to) check some experimental information.

Predicative Construction with the Infinitive

Examples

For-to-Infinitive Construction

The best thing for you to do is to write him a report.

Objective with the Infinitive Construction

I have never seen him studyPhysics.

Subjective Infinitive Construction

My daughteris saidto be a famous scientist.