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ago 用于一般过去时；从现在的以前
before 只能用于过去完成式；是从过去的某一点算起
I went to London three days ago.
She left 30 minutes ago.
Long long ago, there lived a king.
很久很久以前，这里住着一位国王。
Before I arrived at the station yesterday, the train had already left.
我昨天还没到车站之前，火车就开走了
I have never seen her before
I buy a new book for my son every week.
buy sth for sb
I bought my father a coat in Paris last month. = buy sb sth
sell 卖sold-sold
He sold his brother his car for 10,000 yuan=He sold his car to his brother for 10,000 yuan
sell sb sth=sell sth to sb for+money
以…钱卖给某人某物
The goods sell very well.
在英语中，一般是名词或代词做宾语，宾语一般是跟在动词或介词后。
I want an apple.
Put on it!
由一个句子来充当宾语，就是宾语从句
