The first word of the poem, "When," gives us an idea of the structure of the sentence that will follow. The structure is, "When this happens, that happens." As in, "When I broke the glass, I had to find a broom to sweep it up."
But be careful – the second part of the sentence doesn't come until lines 7 and 8. Milton's audience was more used to reading dense and complicated sentences, so you'll want to take the first seven lines slowly. (That's OK, we also think Milton's audience would have had a doozy of a time figuring out text messaging.)
Most readers believe that the poem is clearly about Milton's blindness, but the poem never directly refers to blindness or even vision. Instead, we think that "light" is a metaphor for vision.
The metaphor is complicated. The speaker says that his light can be "spent," and this word suggests that he is thinking of something like an oil lamp. The light is "spent" when the oil in the lamp runs out. To make a contemporary comparison, it would be like someone comparing his vision to a flashlight that runs out of batteries before it is supposed to. Milton is suggesting that he got a bad deal.
The word "spent" also makes us think of money. Milton is reflecting on how he has used or "spent" his vision, now that it is gone. Has he used it wisely, or did he fritter it away because he thought it would never run out?
The word "ere" means "before." How does Milton know that he became blind before his life was halfway over? For this to be true, wouldn't he have to be some kind of psychic who knew when he was going to die? The usual explanation of this line is that Milton guesses roughly how long he will live. Milton went completely blind at the age of 42.
Finally, calling the world "dark and wide" makes it sound like a scary place, doesn't it? Interestingly, Milton makes it seem as if the world has run out of light, rather than growing dark because of any blindness on his part.