Farewell, thou child of my right hand, and joy; My sin was too much hope of thee, loved boy.
The poem begins with the speaker saying "farewell" to his child.
The speaker also says his "sin" (or mistake) was that he had "too much hope" for his son. He implies that there is some connection between his love for the boy and the boy's death. That's odd.
The phrase "child of my right hand" is also kind of funny. It implies that the speaker's son was born from his right hand (weird, huh?). Or maybe this implies something about the speaker's favoritism toward his first-born? (Think of the phrase "my right-hand man.")
"Child of my right hand" also would indicate that the speaker's son is lucky. The right is always the good side; the left is the bad or sinister side. (In Latin, a language Jonson knew well, the word for left side is the same word that gives us sinister in English.)
As it turns out, Ben Jonson's first son was named Benjamin (just like his father), and in Hebrew Benjamin means "son of right hand." Okay, so maybe "child of my right hand" isn't so weird after all. But how many people really know what Benjamin means in Hebrew? Sheesh!
Finally, read these lines aloud. Go ahead. Nobody's looking. Notice anything? These lines are in a very famous rhythm: iambic pentameter. If you've read any Shakespeare (a friend of Jonson's), you should be picking up what we're putting down.
Basically, an iamb is a combination of one unstressed and one stressed syllable. It sounds like da-DUM (for example, the word "belong" is an iamb). Iambic pentameter is five of these bad boys in one line, or da-DUM da-DUM da-DUM da-DUM da-DUM. See how these first lines fit that rhythm almost perfectly? For more on this, check out "Form and Meter."
Seven years thou wert lent to me, and I thee pay, Exacted by thy fate, on the just day.
The speaker continues his lament, saying that his son was only seven years old when he died ("seven years thou were lent to me").
He also says that his son was "lent" to him, and the time has come for him (the son) to return to Heaven. It sounds almost like he checked out a library book or something.
("Wert," by the way, is an obsolete form of "were." You could say to your friend who just left the bank, "you wert just there"—though you might get some funny looks if you did.)
"I thee pay" means that the speaker must now "pay back" his son. In this money-lending metaphor, God, or Heaven, is a bank, or lender.
The phrase "exacted by thy fate" continues that metaphor. "Exacted" means something like demanded, only more in the sense of a punishment. It's like saying, "Since you didn't return your books to the library, we will now exact punishment in the form of 10 dollars." (That's a way harsh late fee, by the way. We wonder who would charge that much.)
"Just" is an interesting word to use to describe the day your son dies, don't you think? Most people don't consider death fair or "just."
The speaker implies, however, that it is "just" or fair for the child to die since he believes that the child was only on "loan" from God. (It's kind of like how it is only fair for you to return your books to the library after the loan-period has expired.)
Notice that the rhythmic iambic pentameter that was so regular in lines 1-2 gets pretty mangled in line 3. It comes back in line 4, though.