Why this sentence
I look forward to hearing from you (gerund)
is correct instead of
I look forward to hear from you (infinitive)
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Sign up to join this communityWhy this sentence
I look forward to hearing from you (gerund)
is correct instead of
I look forward to hear from you (infinitive)
The first sentence is idiomatic whereas the second is not. I wish I could give you a purely logical explanation, but language is not entirely logical.
Many verbs in English are formed from the combination of a root verb and prepositions; "look forward to" is one of those verbs. You need to think of it as an entire verb. It takes an object.
She is looking forward to the concert.
I am looking forward to my wedding.
Do you see that "to" is NOT being used to introduce an infinitive, but rather is part of the main verb? "Look forward to" does not mean that anyone is actually staring at anything. It means that someone is happily awaiting something.
The most common way to form a noun from a verb to use the progressive participle (a so-called "gerund") although sometimes infinitives are used. Whether a gerund is called for as an object or an infinitive is called for as an object depends on the verb. "Look forward to" calls for a gerund. "Expect" calls for an infinitive.