The following are correct:
- I want to eat this.
- I want to! (e.g. in response to a suggestion)
- I forgot to do it.
- I forgot to! (e.g. in response to “Did you do it?”)
However, while the following are correct:
- I look forward to eating.
- I look forward to it!
- I’ve changed my mind about eating.
- I’ve changed my mind about it!
The following are obviously wrong:
- X I look forward to!
- X I’ve changed my mind about!
Why? What’s the underlying rule here? I have a vague idea but would like a proper grammatical explanation since I’m trying to help an English learner who asked me this question. Thank you!