Should I write "wouldn't know if you did" or "wouldn't know if you had" in the following sentence?

Question

Speaker A: "I don't sleepwalk."

Speaker B: "You wouldn't know if you did/had."

Speaker A: "But you would."

Speaker B: "Not if I was asleep."

Should it be did or had? And why?

Answer 1

Like Kman3 said in their answer, which one you need to use is reliant on what is being implied.

"You wouldn't know if you had sleepwalked"

Here, the Past Perfect indicates the sleepwalking has been completed already and was finished before a second past action began.

vs

"You wouldn't know if you did sleepwalk"

Meanwhile, the Past Simple indicates the sleepwalking happened at a specific time in the past and it could have been for either a long or short timespan.

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Since you used "don't" in the first sentence rather than "didn't", keeping it in the Present Simple tense could clarify things for readers further. This works if the conversation discusses something that happens frequently, happens generally, or is a narrative.

A: You don't sleepwalk.

B: I wouldn't know if I do [sleepwalk].

A: But you would [know].

B: Not if I were asleep.

Answer 2

You should use the auxiliary verb did, emphasizing the general action of sleepwalking.

You wouldn't know if you sleepwalk.

You wouldn't know if you did [sleepwalk].

Had is the past perfect, and requires the past participle sleepwalked. If Speaker A was referring to a past event of sleepwalking rather than sleepwalking itself, this would be better.

I didn't sleepwalk.

You wouldn't know if you had [sleepwalked].