1\ask sb for sth

you can ask him for help

2\knock at the door

yesterday a begger nocked at my door.

Did you hear the knock at the door?

knock off

it's pass 6 now so let's knock off

the shop assistant knocked 10% off the bill.

the car knocked the boy over.

3\ in return for this, the beggar stood on his head and sang songs.

in return for=to return the favour.

4\ then he put a piece of cheese in his pocket and went away.

a piece/set furniture.

two pieces cheeses

5\he called at every house in the street once a month. 

call at

call in

call by  i will call by at the shop on my way home.

call on  i shall call on him tommrow .

once

twice a month

6\ 用在可数名次前表数量的修饰词

a(an) ;some; a few ;  a great many ;a great  number of;  lots of.

用在不可数名次前表数量的修饰词

some ;a little ;much; a great deal of; a lot of.

表示计量单位

a glass of milk

two bag of

a bar of

a cup of

a piece of