They’re (They are) contains the subject and verb of a sentence.
Their is a possessive adjective. It means that something belongs to them.
There is an adverb. It refers to a place.
Other sets of words can be equally puzzling. Look at the examples below to see how they differ.
accept (permit, allow, agree with)
except (exclude, but not)
“I can accept all of you into this class,” the teacher said, “except Fred. He’s not ready yet because his English skills are too low.”
“I accept your suggestion to add vegetables to the pizza,” my roommate said, “except onions. I don’t like onions.”
every day (7 days a week)
everyday (common, ordinary)
She’s lucky because every day she can wear her everyday clothing to work. She doesn’t have to wear a suit.
Students who study abroad often get lonely. They might call their family and friends every day to talk about everyday things.
all ready (ready to go)
already (before now)
I am all ready to go because I already packed my backpack.
He already studied this grammar, so he is all ready to move to the next topic.
assure (promise someone something)
ensure (make sure of something)
insure (buy a company policy to pay for sickness or damage)
She insured the package to ensure its safe arrival. The company representative assured her that it would arrive undamaged.
The tour manager assured us that everything possible would be done to ensure our safety while we were traveling. He said that each member of the tour was insured for up to $100,000 in personal damages.
Here are some links for further study: