What is memory in past tense?

Answer 1

'Memory' remains the same in both past, present and future tense.

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Memory is a noun not a verb and nouns do not have tenses. The verb associated with memory is the verb "to remember" the past tense of this verb is "remembered" - "He remembered his last birthday".

Alternatively one could say "he had a memory of his last birthday" which puts the memory into the past.