Sure, if that long with love-acquainted eyes Can judge of love, thou feel'st a lover's case;
The speaker now proceeds to, essentially, answer his own question. He wonders whether the moon is feeling the pains of love, and decides that he, in fact, is.
The sentence is a little tricky so we'll take it bit by bit. "Sure" is pretty much just "surely," while "long" means "for a long time." "That" is a shortened form of "[it is true] that."
"Love-acquainted eyes" means exactly that: eyes that are acquainted with love, that know what it looks like, that can recognize it anywhere.
So, putting this all together we get the following: "Surely if it is true that eyes that have long been acquainted with love can judge in matters of love, you definitely are experiencing a case of love."
Sheesh, that's a lot of work for one sentence, don't you think? But notice how much neater, tighter, and compact the speaker's way of saying it is. And you thoughts poets were all about being long-winded and over the top. Hah.
We mentioned earlier that, in the old days, poets liked to end words with "-est" or "-st." We have that again here with the word "feel'st," which should be pronounced as one syllable. Why? Again, if it's not pronounced as one syllable, the iambic pentameter doesn't work.
The speaker sure is intent on personifying the moon and turning it into some kind of love-sick, heavenly body. Knowing what we know about Astrophel and Stella (the sonnet sequence from which this poem is taken), it's pretty clear that the speaker, Astrophel, is projecting, and we don't mean showing a movie.
Well, we sort of mean that. Click here to find out what projecting psychologically means. Think it applies to our speaker?
Yeah, us too.
I read it in thy looks; thy languisht grace To me that feel the like, thy state descries.
This sentence is another toughie, so we'll take it slow. It starts out pretty easy, though: the speaker can "read it" in the moon's looks. What he reads is the fact that it (the moon) is—wait for it—love sick.
He can tell because the moon has a "languisht grace." "Languisht" is an old spelling of "languished," and it means weak or lacking energy. "Grace" here just refers to the moon's style of movement, the normally graceful way it ascends into the sky.
Remember, the moon is climbing the sky very slowly, with very sad steps, and its face is "wan."
The speaker can tell the moon is suffering a "case" of love because his "languisht grace" illustrates ("descries") this "state" to "me [our speaker] that feel the like."
So, to sum up: the sentence says "Your lack of energy shows your state to me, because I'm feeling just the same."
Just like in the previous two lines, the speaker here gives us a compact, tight, but tricky sentence. The syntax is funky too (note that "descries" comes at the end, just as "tries" does in line 4).
It's also clear that the speaker is feeling the same thing ("the like") and is thus probably projecting his own feelings of love-sickness onto the moon—who, you know, isn't a person and doesn't experience human feelings.
So we've come to the conclusion of the eighth line, which means it's high time we check up on our rhyme situation. It goes like this: ABBA ABBA. Hmm. We've got two quatrains (groups of four lines) that have the same rhyme scheme—interesting.
Head over to "Form and Meter" for more on this stuff. For now, though, this repetitive rhyme scheme seems significant. After all, this poem is all about doubleness. The moon is the speaker's double, his bro, his twin, his companion, his other half—a heavenly body that he (the speaker) thinks is experiencing the same exact feelings.