Infinitives are used in a common sentence pattern:
ask somebody to do something
I asked my brother to get me a dictionary.
want somebody to do something
The professor wanted John to come to his office.
expect somebody to do something
Her parents expect her to be a doctor.
For some verbs, we don’t use it this way. For example:
I let him borrow my bike. (correct)
I let him to borrow my bike. (incorrect)
Here we use an infinitive without “to.” It is called a zero infinitive. There are a few verbs used this way.
Dad let me drive his car.
Let’s go shopping together.
The mother made her son clean his own room.
The movie made him cry.
I had my brother carry my suitcase.
I saw the thief run away.
You can also say “see somebody doing something,” but you don’t say “see somebody to do something.”
I heard her sing in the room.
My friend helped me fix my car.
My friend helped me to fix my car.
“Help” is a special verb. It can be used with or without “to.”