問兩個簡單的語法問題，希望高手指點。。。

1。瘋狂英語365中有一句 All I have to do is learn English.
     為什麼不是 All I have to do is learning English.

2.  新概念2冊2課有一句話 You have been here only a week.
     為什麼不是 You have been here only for a week.

All I have to do is to learn English;

You have been here for only a week.