1-

Never say: “After the bell rang, the boxers continued hitting themselves.”
Themselves means that one person is doing something to himself or herself, and the other person is doing something to himself or herself,
 e.g. ‘ the children washed themselves and got into bed.’

You should say: ”After the bell rang, the boxers continued hitting each other.”

If both are doing something to the other person, you use each other.
 

2-

Never say: “Three men stole a bank in North London yesterday.”
You can only steal things that you can move, because it means that you take it away from the place or person it belongs to,
e.g. ‘ He stole some food from the supermarket.’

You should say: ”Three men robbed a bank in North London yesterday.”
If you talk about the places ( e.g. banks and post offices) that people steal things from, you use rob.
 

3-

Never say: ”There were very little people in the supermarket this morning.”
Little is the opposite of much, and is used with uncountable nouns, e.g. ‘ I have very little time.
If you use little with plural noun, it means small
e.g. ‘ look at those lovely little houses.

You should say: ”There were very few people in the supermarket this morning.”
People is a plural noun, e.g. 'the people are very friendly here.' With a plural noun you have to use a different word which means the opposite of many.
 

4-

Never say: ”The landlady I stayed with in England was a very good cooker.”
A teacher teaches, and a writer writes, but a cooker is not the person who cooks. A cooker is a machine which cooks, just as a dishwasher is a machine which washes dishes.

You should say: ”The landlady I stayed with in England was a very good cook.”
For the person who cooks, you just use the verb as a noun.
 

5-

Never say: ”Helen usually goes to work with her car.”
If you went somewhere with your car, it would be the same as going with another person – it would accompany you.

You should say: ”Helen usually goes to work by car.”
If you prefer to sit inside your car when you’re traveling, use a different preposition. Also, you use just car, without her or my, etc.
 

6-

Never say: ”I’m very boring in this class.”
If someone or something is boring, it’s causing the problem. You don’t usually tell people that you are boring! You might say that a lesson is boring (or interesting).

You should say: ”I’m very bored in this class.”
Here you are describing how you feel when you think another thin g is boring.
 

7-

Never say: ”Your plants have grown up a lot since my last visit.”
People grow up when they get older and stop behaving like children. When you grow up you start to do adult things.

You should say: ”Your plants have grown a lot since my last visit.”
Here you are talking about something getting bigger. You don’t need a preposition for this.