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It's common to hear people say "used to" to indicate that they did something in the past but no longer do; for example, "I used to play basketball." How would "used to," used in that context, fit into a sentence diagram? What part of speech is it?

It is common to hear people say "used to" to indicate that they did something in the past but no longer do; for example, "I used to play basketball." How would "used to," used in that context, fit into a sentence diagram? What part of speech is it?