Wherefore dost thou forget us, etc.? The poet does not say," Wherefore hast thou forgotten us?" One of the psalmists, indeed, does go so far (Psalms 74:1); but the poet of this lamentation, with a more tender and trustful reserve, adopts the tense of feeling (the imperfect) in preference to that of fact (the perfect), and asks, "Wherefore dost thou [to my feeling] forget us?
Wherefore, if Jehovah's power is still unbroken, does he allow Israel to feel herself forsaken?" The fact is certain, viz. that the land of Israel is desolate, and (the poet seems to imply) desolate for some time already.
The interpretation is hypothetical, and, as the last verse will show, the poet cannot bring himself to believe that it can be accurate.