They’re, Their, There
They’re is a contraction of “they are”, so it should only be used in situations where “they are” would also be acceptable.
Example: They’re coming later on, after work.
Their is the possessive form of “they.”
Example: Their dog is massive.
There is a pronoun referring to a specific place at some distance from the speaker.
Example: I’ll meet you over there.
Mega-example: They just called to say they’re running late, but I think I see their parked car over there.
The same basic principle applies to the equally frequently confused pair your/you’re: your is possessive, and you’re is a contraction.
Example: You’re not supposed to be here.
Turn your car around and get out!
Too, Two, To
Too is an adverb that is a synonym for “also.”
Example: Where are you going? I want to come, too!
Two is a number.
Example: There are two sides to every story.
To is a preposition. Therefore, it’s the only one of these three that can be used in a phrasal verb.
Example: We’re going to the movies later.
When I came to (=regained consciousness after passing out), I was surrounded by doctors.
Stop doing vs stop to do
This isn’t technically a homophone, but rather two phrases based around the same root verb. Nevertheless, it’s an important distinction to know, and I’ve been hearing these words used incorrectly with increasing frequency. If you stop doing something, you don’t do it anymore. If you stop to do something, you interrupt another activity in order to do something.
Examples: I stopped playing violin when I was 8 years old. I hated it.
On the way home from work, I stopped to get a turtle out of the road.