作定语的成分
不定式作定语：
The boy to write this letter needs a pen./写这封信的男孩需要一支钢笔.
分词（短语）作定语：
The smiling boy needs a pen bought by his mother./那个微笑的男孩需要一支他妈妈买的钢笔.
The pen bought by her is made in China./她买的笔是中国产的
动名词作定语：
the girl sitting under the tree is my sister
请问在什么情况下用什么作定语,