Classifying a grammatical form reveals underlying assumptions about what grammar actually is. Classifying "used to" demonstrates this point.  

For example, if  grammar is understood as identifying differences in meaning then Craig's answer makes perfect sense.

>>> [log in to unmask] 08/09/05 7:53 AM >>>

     Here's a quick answer before>I head off to teach./  "Used to" I would classify as a modal auxiliary, somewhat similar to "would", but
implying that the pattern is no longer operative. 

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However, I think grammar (more accurately, syntax) is separate from meaning and, from that perspective, used to lacks the crucial grammatical properties of prototypical modals.  Modals (I recognize some defective modals with ought to and must), and all other helping verbs have three core properties.  They "move" for yes-no questions (1).

1)  Can you speak English?

Not follows them.

2)  You can not speak English?

And, they are repeated in tag questions?

3)  You can speak English, can't you?

One of the key properties of main verbs, when they occur without any helping verbs, take do-support.

4a)  You didn't use to speak English.
  b)  *You used not to speak English.

Consider the tag question.

5)  You used to speak English, didn't you?

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I would be very, very reluctant to define an auxiliary this way.

> It's an auxiliary in that it can't be a main verb. (Here, it goes with
 > "love.")   

In French and German, the equivalent of the word for "want" is a modal, that is not the case for English want.  "Want" in English has none of the syntactic properties of English modals.

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