Another example of frequently confused homophones (words which sound the same but have different spellings and meaning) are two, to, and too.
Two is a number:
Ex.: Two days.
Preceding a noun, to can be used as a preposition:
Ex.: I am going to the store.
When it precedes a verb, to indicates its infinitive:
Ex.: I like to dance.
Ex.: I need to eat.
Too can be used as a substitute for 'also.'
Ex.: I want to go, too. (I want to go also.)
Too, when preceding an adverb or adjective, can also mean excessively:
Ex.: He is walking too quickly.
Ex.: I'm too tired.
If you're referring to something excessive, use the word 'too.'
To denote the number 2, use the word 'two.'
For everything else, 'to' is appropriate.