Use to for a direction or before a verb (go to school, I want to sleep), too when you mean "also" or "more than enough" (me too, too hot), and two for the number 2 (two cats). This article explains each word with examples.
Homophones are words that sound the same but have different meanings, and often different spellings. They cause problems for students and writers of English all over the world. One set of homophones that is especially easy to mix up is to, too, and two.
All three words sound the same and are almost spelled the same. A spell checker that looks up one word at a time cannot flag them, because all three are real words. Two is the easiest of the three to tell apart, but all of them are worth a closer look.
To
To usually points toward something: it shows a movement or a direction toward a person, a place, or a thing. It also comes before a verb, as in to think or used to, and it can show an intention, a purpose, or a result.
Some examples of the correct use of to:
He came to our aid when he saw we were in distress.
Did she come to our house last night?
And to think that I used to really like him at one point!
Have him call me when he comes to his senses.
Please come back to me, my love!
Too
Too is an adverb with two meanings. First, it describes an addition: it is another way of saying "as well", "besides", or "also". Second, it describes an extent or a degree that is more than enough, or more than wanted.
Some examples of the correct use of too:
It is too bad that Harry never bothered to apply for that job.
You would be delighted too if he had given you such a nice gift.
I tried talking to her, but she did not seem too interested.
Lisa wanted to work out, but she was too tired to run.
Two
Unlike to and too, the word two is easy to tell apart from the others. Because it stands for a number, it is hard to confuse. It also appears in the phrase in two, which means into two parts.
Some examples of the correct use of two:
The last two people off the bus will have to wait in line longer.
These two items are identical but do not cost the same.
I cut the orange in two: one half for me, the other for you.
Yes, you are right. Two is better than one!
How to remember the difference
People confuse to and too with each other much more often than with two, because they are so close in spelling. There are two ways to learn homophones like these. You can learn the easiest word first: once you are sure of it, it is easier to tell the other two apart. Or you can concentrate on the word that gives you the most trouble. Once you use that one correctly, the others become easier too.
A common memory aid: too has an extra o, just as it means something extra.
To learn more about this kind of word, read the difference between homonyms, homophones, and homographs. Another pair that trips up many writers is theirs and their's. Because all three words are spelled correctly, a real word in the wrong place is a job for the AI Spell Checker, which reads whole sentences and catches more of these mix-ups.