Even the best of us can be left scratching our heads at some of Milton's lines. Our guy is known for his crazy syntax (that's a fancy word for sentence structure), requiring us readers to go hunting for the subject and verb with a flashlight and, perhaps, a shovel.
People have often characterized Milton's verse as Latinate, meaning that he writes English as if it were Latin. What does that mean, exactly? Well, in Latin, word order doesn't matter as much as it does in English, and verbs tend to come at the end of sentences. Sound familiar? Milton is always writing things out of order, and sticking the verb in at the tail end of the sentence.
Take these lines for example: "Thee, Shepherd, thee the woods, and desert caves, / With wild thyme and the gadding vine o'ergrown, / And all their echoes mourn" (39-41). Notice how the object of the verb comes first ("Thee, Shepherd," i.e. Lycidas), then the subject of the verb ("the woods, and desert caves"), then a description of that subject ("with wild thyme," etc.), and finally the verb ("mourn"). All it's saying is that the woods and caves (that are overgrown with thyme and vines) mourn for the shepherd. In other words, the sentence is sort of backwards, but in a pretty way. That's classic Milton.
There are a number of examples of this. Take lines 25-27: "Together both, ere the high lawns appeared / Under the opening eye-lids of the morn, / We drove a-field." Normally we might say, "We drove a-field together before the high lawns appeared," but Milton says, "Together, before the high lawns appeared, we drove a-field." Again, the verb comes at the end. As you read through "Lycidas" you'll notice lots of examples of this strange Miltonian syntax.