Sometimes I get the feeling that some of my ESL students (including advanced ones) believe that there are a limited number of “who” and “which” out there, and they are afraid of using them all up before they die.
The problem happens when students are trying to write more advanced styles with a dependent and independent clause in a sentence.
Some examples:
Mistake: The people are walking their dogs should keep them on a leash.
Correction: The people WHO are walking their dogs should keep them on a leash.
Mistake: I try to give money to charities help homeless people.
Correction: I try to give money to charities WHICH help homeless people. *
I’ve also notice that this mistake often happens when students start a sentence with “there”.
Mistake: There was an accident happened near my house.
Correction: There was an accident WHICH happened near my house. *
* We could substitute the word THAT for WHICH in these sentences.