Mercy come rain down on me (what tense?)

English as a Second Language English Vocabulary, Grammar and Idioms
1

“Hundreds of tears on the water
Mercy come rain down on me”

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What tense is being used? Is that simple present tense?

2

I think it is intended to be an imperative sentence, but there are mistakes in the grammar of those lyrics (‘I’ve walk in and out of the fire’??) and little punctuation, so my answer is tentative.