Here are some of the most common mistakes we use in English :
1- there - their -they 're:
There: trefers to a place " There are 25 students in my classroom".
Their: a possessive adjective "their car is new ".
they're:abbreviation of they are " they're leaving now".
2-It's - Its :
It's :It is "It's time to go"
Its: a possessive adjective " they painted their house and changed its color".
3-Whose - who's:
whose :is a pronoun " whose pen is this ?"
who's:an abbreviation to who is ,or who has " who's been to London?"=who has
"who's at the door?"=who is
I'll add some more coomon mistakes here ...and If you have any common mistakes or questions about that just reply here and let's share !!