动词, 不及物动词, 及物动词
A 有些动词只是及物动词; 它们不可以单独用，后面必须跟宾语。
{ False: They always want after lunch.
Right: They always want a cup of tea after lunch.
{ False: He is sending now.
Right: He is sending a letter now.
B 有些动词只是不及物动词；它们可以单独用，如果后面想接宾语，动词后面必须加上介词。
{ Right: He is looking around.
False: He is looking me.
Right: He is looking at me.
{ Right: He is listening carefully.
False: He is listening the teacher carefully.
Right: He is listening to the teacher carefully.
C 有些动词既是及物动词，又是不及物动词; 但是有时候词义会改变。
{ The customer is asking loudly.
The customer is asking for you now.
The customer is asking a question now.
{ If you work hard, you will succeed.
If you work hard, you will succeed in passing the exam (same meaning).
If you work hard, you will succeed John as the manager of this company (different meaning).