did is used because we are talking about a time in past when headaches started. get should be used in gerund because it has been used after the verb start. after some verbs gerund should be used, start is one of them.
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AnonymousWhen did you start getting headaches (them)?
In this question why they use "did", while the verb is "get"."Getting" in this case is the non-finite gerund.
The main finite verb is "to start."
We can always optionally add "do support," or the auxilliary verb "to do" to a declarative sentence.
"I often [do]