英文 grammar?

(I) gerund (verb + ing) :
　- can be used "after certain verbs"
　　eg. I enjoy cooking.
　　　　(verb) (gerund)
　　eg. The crowd kept moving forwards.
　　　　　　　 　(verb)(gerund)
　　*eg. I went shopping yesterday.
　　　　 (verb) (gerund)

(II) to + infinitive :
　- can be used to SHOW PURPOSES
　eg. He came to visit his uncle this morning.
　　　　 (verb)(　 purpose　　)
　 eg. I went to see a doctor yesterday.
　　　 (verb)(　 purpose　)
　 *eg. You must 　go　 to get your homework now.
　　　 　　　　 (verb) (　 　　purpose　　　)

(III) bare infinitive (infinitive without 'to') :
　- can be used :
　a) after 'modal verbs'
　eg. I will meet you tonight.
　　　　　^^^^^
　b) after 'let', 'make' and (sometimes) 'help'
　eg. My parents let me go abroad.
　　　　　　　　^^^
　c) after 'verbs of perception' --- see, watch, hear, notice, feel, sense
　eg. I saw it walk away with a piece of meat in its mouth.
　　　^^^^
　d) after 'Why'
　eg. Why go out the night before your exam?
　　　^^^

The verb "go" does not belong to any of the above (a,b,c or d)
✪ ∴ "go" 後面不會用 bare-infinitive ！

Conclusion:-
=========
1. I went shopping yesterday. (✓) -- Type I ( go + gerund )
2. (a) I went to see a doctor yesterday. (✓) -- Type II ( go + to-infinitive )
　(b) You must go get your homework now. (X)　Should be :
　　 You must go to get your homework now. (✓) -- Type II ( go + to-infinitive )
3. "go" 後面不會跟 bare-infinitive ！
　　　　　　　　　　　

Difference uses of intransitive verb "go":

1. travel to a place in order to take part in an activity or sport:
go (for) something
~ I went shopping yesterday. (correct)

2. travel from one place to another:
go to do something
~ I went to see a doctor yesterday. (correct)

3. move from one place to another, to show purpose or tell somebody what to do:
go (and) do something
~ You must go get your homework now. (correct; with "and" left out but not bare-infinitive)