The marker to is normally used before infinitives. Note that this to is not a preposition; after the preposition to we use –ing forms.
The to-infinitive has many functions.

Infinitive as subject

An infinitive can be the subject of a sentence. This was fairly common in older English.

• To err is human, to forgive divine.
• To find fault with others is easy.

In modern English, this is unusual. We more often use it as a ‘preparatory subject’, and put the infinitive clause later.

• It is human to err.
• It is easy to find fault with others.

Infinitive as object

Many verbs can be followed by an infinitive clause in the place of the direct object.

• She wants some exercise. (noun object)
• She wants to dance. (infinitive object)
• I don’t want to go to bed.

Infinitive clause as complement

An infinitive clause can be used after be as a subject complement.

• The best thing to do now is to vanish.
• You are to sit in that corner and keep quite.
• The main thing is to stay calm.

To-infinitives can be used as the object or complement of a verb, adjective or noun.

• You have the right to remain silent.
• She was anxious to contact her husband.
• That was a sight to see.
• Automation has come to stay.
• It is time to go.