Everyone learns grammar in elementary school, and yet 12 years later people are still making mistakes. The most popular examples of this are using the right form of 'to' and 'there' when writing.

When to use to, too, and two

People mix up to and too a lot, mainly they use to when too is necessary. Too is used to describe an abundance or excess amount of something
There are too many people here.
There was too much work to finish in one night.

Two is the simplest form to use. It's the number 2.

She's raising two kids.
It takes two to tango.

When two or too does not apply, use to

I have to walk the dog.
I need you to help me with something.

There, Their, and They're

The biggest mistake I find people making under this set of words is using there when their is the proper choice. Use their when you are describing ownership of something.
The car belongs to their dad.
It used to be their house before they sold it.

They're is simply a contraction of they are.

They're going to the gym later.
They're here to say goodbye.

For any situation that doesn't fit the criteria of their or they're, use there

There is laundry to do.
How many apples are there?
I hope this helps.