ben johnson-on my first son?

Question

explain the speaker-“Seven years thou wert lent to me, and I thee pay, / Exacted by thy fate, on
the just day”

Answer 1

the poem is about his son's death:
"seven years thou wert lent to me" - he was seven years old when he died.
"and i thee pay,/Exacted by thy fate, on the just day" - he (Jonson) is paying the "price" of "borrowing" his son ("thou wert lent to me").. the price is his agony in his son's death. and the "just day" is the day his son died.

or at least that's how i see it

one of my favorite poems.
hope this helped