Let is followed by an object + infinitive without to.

Let him go. (NOT Let him to go.)

I won’t let you see what I am doing. (NOT I won’t let you to see what I am doing.)

Let her do what she wants.

Let him marry the girl he loves.

Let them go home.

Let can also be followed by an adverb particle.

She didn’t let me in.

Expressions with let

To let somebody know is to tell or inform them. To let somebody have something is to send or give it to them.

Let me know your thoughts.

Let me have a look at the reports.

Could you let me have the bill for the repairs?

Let somebody down

To let somebody down is to fail to help him in time of need.

A true friend will never let you down.

Let oneself in for (= cause oneself to be responsible for something troublesome.)

Let a secret out

To let a secret out is to share it.

Let off

Let off has two meanings:

1) fire or discharge a gun etc.

2) allow somebody to go unpunished or with slight punishment.

Let is not normally used in the passive voice. Instead, we prefer verbs like allow.

We let them go home. (Active voice)

They were allowed to go home. (Passive voice) (NOT They were let to go home.)

Let somebody / something alone (= leave undisturbed)

Let me alone.

Let alone can be used as a preposition. It means ‘not to mention’.