Milton loved the classics, and in the 17th century, "classic" meant anything associated with Ancient Greece or Rome. The heart of the Roman Empire was located in what is now modern-day Italy, and the sonnet was invented in Italy, so it was not a surprise that Milton would favor the original Italian form of the sonnet. This form is divided up into two sections, one with eight lines and one with six. Shakespeare, on the other hand, used a sonnet form that ended with a rhyming two-line couplet. The Italian sonnet form was made popular by the Italian poet Petrarch, who was to the literary Renaissance what The Temptations were to Motown.
The rhyme scheme of this sonnet is ABBAABBAC CDECDE. So, you can see that lines 1, 4, 5, and 8 all rhyme with each other. Unlike a classic Italian sonnet, "When I consider how my light is spent" does not divide cleanly into eight lines and six lines, however. The first section of the poem consists of the speaker trying to frame his foolish question, and the second consists of the response to the question by a figure named "patience." Most Italian sonnets have a sharp thematic turn or "volta" between the two sections, but in this poem the turn is a bit muddled between lines 8 and 9. If you think about it, the confusion makes perfect sense, as it conveys the awkwardness of someone (patience) interrupting someone else (the speaker) before the speaker can say something stupid.
The meter of the poem is classic iambic pentameter, with five iambs (an unaccented syllable followed by an accented syllable). Some of the lines do not fit the pattern exactly, but the pattern itself is clear:
"Doth God ex-act day-la-bor, light de-nied?"
Finally, this poem features a lot of enjambment, which is when one line runs over into the next without a pause. Just check out the end of each line, and you'll find that over half lack punctuation markers like periods or semi-colons.