Look forward to

Why is the bare form of the verb hear unacceptable in the sentence below?

(1) *I look forward to hear from you.

This student was confused by the grammar rule that requires the infinitive after the word to, such as in the following sentences:

(2) They decided to stay for another day.
(3) We hope to make the trip next month.

The to in sentences (2) and (3) is used before a verb to show that it is the infinitive. It is not a preposition.

The to in look forward to, on the other hand, is a preposition. This makes look forward to a phrasal verb.

As with most phrasal verbs, what follows the preposition is a gerund, not the infinitive:

(4) I look forward to hearing from you.
(5) He was accused of stealing her money.
(6) They insist on giving us a lift.