动词后可以加介词，也可以加副词，这是为什么？——原因可能是多方面的。如：

▲不及物动词如果要后接宾语，它应该带上合适的介词。如：

He is looking for his keys.

He is looking at the blackboard.

He is looking after his sister.

这里加上介词 for, at, after，是为了让 look 这个不及物动词能带上宾语。

▲有时动词后接介词可能是为了表达某种意图，如时间、地点、原因、目的等。如：

He gets up at six. (at 表时间)

He lives in Beijing. (in 表地点)

He failed because of his illness. (because of 表原因)

He went for a swim. (for 表目的)

▲动词后接副词可能是为了表达某种意图。如：

He spoke slowly. (slowly 表方式)

He moved away. (away 表方向)