A and An
If the word that comes after a or an starts with a vowel, then you will use an, if it doesn't, then you use a.
Example:
This is a cat
This is an ice cream
Vowels: A, E, I, O, U
Affect and Effect
You will use affect as to influence something (as a verb) but you'll use effect as something that has already been influenced (as a noun.)
Don't let this affect your future plans.
This has effected my life.
Then and Than
You'll use then when you are using time words, but when you use than its comparing something.
Then, we went to the park.
This is better than that.
Who and Whom
Who is to act and talking about a he/she/it but whom is to receive.
To whom it may it concern.
Who is that kid?
It's and Its
It's is also said as it is, or it has, but its is showing possession (usually him/her.)
It's (it is) all good.
The cat fell on its side.
Who's and Whose
Who's is short for who is, while whose shows possession
Who's (Who is) at the door?
Whose been knocking?
