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"I haven't seen her."

"I didn't see her."

Does it matter whether "have" or "did" is used? Are they interchangeable in this sense or is there a slight difference?

Kind of confused here.... a matter of choice?

Thanks for the help.
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Ter"I haven't seen her."

"I didn't see her."

Does it matter whether "have" or "did" is used? Are they interchangeable in this sense or is there a slight difference?
As explained above, there is a difference.

"I didn't see her" tells us what happened at some particular time in the past. There was a failure to see her at some particular time.

"I haven't seen her" tells us the situation during the entire period from some indefinite time in the past up to the present moment.

CJ
Comments  
It depends on the situation. For example: "I didn't see her at the party." Or: "I didn't see her yesterday." This usage is for a specific time or place in the past.

Or: "I haven't seen her in a week." "I haven't seen her at school lately." This usage is for a duration of time.
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 CalifJim's reply was promoted to an answer.
In this sentence either one is correct depending on your meaning. Have not is any time in the general past whereas did not is in a particural place or time .

I have not seen her at the store. This means I have not seen her at the store at all in the past.

I did not see her at the store. This means when i went to the store i did not see her.