In these two sentences:
- I look forward to get.
- I look forward to getting it.
Why is the first sentence incorrect? When do we use to as an infinitive marker?
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"Look forward to" is a transitive verb phrase, so it wants a noun for its object. Since "get" is not a noun, it sounds wrong. "Getting it" is a noun phrase that satisfies expectations created by the "look forward to" phrase.
The infinitive verb form may sometimes be used as a noun, but the nuance is more of the abstract idea of the verb ("I like to play.") rather than something specific that someone would be looking forward to. Additionally, having the same word repeated (* "I look forward to to play.") does not sound as natural as alternatives ("I look forward to playing.").