"Use to Vs. Used To"

A common mistake both for learners of English as well as native speakers is the difference between “Used to” and “Use to” and when (or if) we put the letter D at the end of the verb.

First let’s look at the meaning of it:
Used to = an action or habit that was common in the PAST but not anymore.

This should not be confused with
To be used to = accustomed to something (normally out of habit or experience)

Notice how Used to ends in –ed which normally means the verb is in the past tense.

• I used to drink coffee every morning but now I drink tea.
• She used to live in London but now she lives in New York.
• He used to play football professionally but he had to quit because of an injury.

These sentences refer to a past habit or a continuous action in the past and they are affirmative.

BUT…

Used to in Negative

If we want to make a negative sentence, the didn’t part shows that we are talking about the past tense.
It is NOT common to use Used to in negative form though we will show you how to do it anyway.

Compare the normal past tense:

• I played rugby on Saturday. (past - affirmative)
• I didn’t play rugby on Saturday. (past – negative – play doesn’t have the –ed at the end because the auxiliary didn’t tells us that the sentence is in the past tense)

And now let’s see a sentence with Used to:

• I used to like her, but now I hate her.
• I didn’t use to like her but now I think she is great.

Many times we use never instead of didn’t. In this case we use “used” in past tense. Compare:

• I didn’t use to smoke.
• I never used to smoke.

Used to in Questions

As with normal questions in the past tense, we use the base form of the verb (not the verb in past tense), since the auxiliary DID shows us that the question is in the past tense.

• +  I used to ride a bike to school every day.
• -   I didn’t use to ride a bike to school. I would normally go by bus.
• ?  Did you use to ride a bike to school when you were a child?

Some more examples

• What did you use to do after school when you were a kid?
• Where did you use to go on vacation with your parents?

Why do people sometimes write Use to instead of Used to in English?

Well, when we say used to, the final D in the verb used is pronounced like a T and since the next word (TO) also begins with the T sound, it seems like there is only one T (that belonging to the word TO).
When speaking, we frequently run words together (join words) so it would sound like Use-to.

Used to and Would

Sometimes we can replace Used to with Would.

• My grandfather used to do exercises every day before work.
• My grandfather would do exercises every day before work.

Surce: http://www.grammar.cl/rules/used-to-use-to.htm