The Infinitive is the base form of the verb:
be, have, walk…
Often the infinitive is introduced by the particle, to:
to be, to have, to walk…
Note that when the infinitive is on its own it’s sometimes known as the Bare Infinitive; when it’s with to then it’s sometimes known as the Full Infinitive.
Full Infinitive Usage/Infinitive Clauses
We use the full infinitive (the infinitive with to) as a noun, adjective or adverb phrase. They are sometimes known as Infinitive Clauses.
To be or not to be.
To put it bluntly: you’re wrong!
To pay for the mortgage, he took a second job.
I want to go to Japan.
A common question and answer expressing purpose is like this:
Statement: I’m packing my bag.
Question: Why now?
Answer: To save time later.
Bare Infinitive Usage
We use the bare infinitive in a number of different ways.
1) After most modal and auxiliary verbs:
{modal/auxiliary} + {infinitive}
We should + go now.
They must + arrive soon.
Note that the main exception here is the modal ought to.
2) With some verbs of perception:
{verb of perception} + {object} + {infinitive}
I watched it explode.
I heard them shout.
Double Infinitive
Sometimes we need to use more than one infinitive in a row:
A picture can be useful to help to explain what happened.
Although correct grammatically, this can sometimes sound a little awkward and we can say instead:
A picture can be useful to help and explain what happened.
Alternatively, we can say:
A picture can be useful to help explain what happened.
These are all correct.