an adverbial verb (past tense)
Memoed
"I see that you memoed the account properly"
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Memo (a contraction of memorandum) is a noun. Nouns do not have tenses. "Memo will a associated with a verb (I wrote a memo) which may show the tense. In modern "business-speak", it is increasingly common to find people using nouns as verbs. Such as "will you memo me?". So presumably you would also get "I memoed him yesterday". See the related link.
Stealth is not a verb. Therefore it does not have a past tense.
it´s not a verb so there is no past tense.
Mercy is a noun, not a verb, so there is no past tense. You could say 'had mercy' which would be in the past tense.
Mercy isn't a verb, so it doesn't have a past tense.