"Didn't use to" vs. "didn't used to"

English as a Second Language English Vocabulary, Grammar and Idioms
1

Hey my great teachers here!!

As I have read one book I have individually found a strange usage of USE TO structure. They are:

1, He didn’t [color=red]used to come here very often.
2, He didn’t [color=red]use to come here very often.

Is there any different between the both? Why after [color=red]used to is used after auxi [color=blue]didn’t?

Thanks beforehand!!!

soklong

2

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Basically, they are spelling variations. Both forms are common and casual.
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3

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Look in Swan’s Practical English Usage, Jamie, under ‘used + infinitive’.

Oops! Where’d he go? Curiouser and curiouser…
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