請問英文 to + gerund 的用法

正如你觀察得到，to之後未必要用infinitive，因為許多情況下，只要to是preposition，那時就要用名詞或動名詞，如：
I was used to studying late.
He devoted himself to writing.
若要想知某個動詞後面的to應該用infintive或gerund，只要我們先將一個名詞放在to後面，而文法仍然正確，那個to後面就是用動名詞，如：
He told me to market.(就不合文法，所以這to要用infinitve，像：He told me to go)
而I am looking forward to your letter.（文法是正確，所以可用動名詞）

希望幫到你！

Some verbs need to follow by 'to infinitive'
some verbs must be followed by gerund.
There is another question concerning gerund being asked in Yahoo knowledge before and the answer is more complete than the above 2 answers.
Please search the question by yourself.

可能 hahatse 君解釋得未够清楚，我在此畫蛇添足。
gerund 之前的 to係用作 preposition, 唔係 infinitive，唔好混淆。所以 個 verb + ing當名詞用，而佢之前phrase嘅 verb (or verb to be)有些一定要加 preposition 才可以 (這是要多用才明)。 如 refer to，look forward to，commit to，devoted to ，is equal to，object to，subject to，amount to，is used to，is accustomed to，is opposed to，with a view to等，to 後面嘅係 gerund，唔係 verb (infinitive)。