幾時要用to同-ing

應是 : I want your help to buy me a present.


I want.............s + vt .....我想....

your help........object=noun.....你的.....幫忙.....


to buy me a present.....( describe to your help )...to buy=infinitive.

infinitive= 未去做,稍後才做.

2011-05-28 23:16:49 補充：
verb 幾時用 v~ing ? =現在分詞

當一組 [ 帶動作的短語 ] 在句內作修飾用時, participle 分詞就要出動.

例 : I saw your mother [ buying a present ] last night. 分詞+賓語,構成 [ 分詞短語 ].


另一種v~ing = 動名詞,加上賓語或修飾語 , 構成gerund phrase=動名詞短語,去作noun用

as subject, object, complement, and used in prepositional phrase.

I think the best way to write this sentence is 'I want you to help me buy a present.' since help is a bare infinitive which does not need a to-infinitive after it.