When I consider how my light is spent, Ere half my days, in this dark world and wide,
The speaker thinks about how all of his light has been used up ("spent") before even half his life is over. As a man without light, he now lives in a world that is both "dark and wide."
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.
And that one Talent which is death to hide Lodged with me useless, […]
These lines are the trickiest in the entire poem, because they appear to be simpler then they are.
The key word is "talent." You probably read "talent" and think of skills like throwing a perfect spiral or being a piano prodigy. But there's a double meaning intended for people who know history or Biblical scripture. In the ancient world, a "talent" was also a standard of weight used to measure money, just as a "pound" is a measure of both weight and currency.
You can read Matthew 25 (it's short), but here's our brief summary of "The Parable of Talents." A lord gives three of his servants some money ("talents") to hold on to when he leaves for a trip. Two of the servants use the money to gain more money for their master. (In contemporary language, we'd call this 'investment.') But the third servant just buries the money, the ancient equivalent of hiding it under your mattress. When the lord returns, he's happy with the first two servants and gives them more responsibilities, but furious with the third servant. He exiles the third servant into the "darkness," which is the equivalent of "death."
When Milton says that talent is "death to hide," he is referring to the money in the Biblical story and also to his own "talent," in the sense of a skill or trade.
There is no way to tell what specific talent he means, but our guess would be his intelligence and his writing and reading skills, which he had used in service of Oliver Cromwell's government. This "talent" is "lodged" or buried within the speaker just like the money in the story. It cannot be used to make greater profit.
[…] though my Soul more bent To serve therewith my Maker, and present My true account, lest he returning chide;
The speaker has just told us that his talent is as useless as money buried in the desert, but now he says that his uselessness has nothing to do with a lack of will. To the contrary, his soul desires (is "bent") to use his skills in the service of his "Maker," God.
When he is faced with God, he wants to have a record of accomplishment to show Him.
God is being compared with the lord from the "Parable of the Talents" in Matthew 25. When God "returns" to him like the master in the parable, the speaker wants to show that he has used his talents profitably.
The word "account" here means both" story" and "a record of activities with money."
If the speaker turns out to have wasted his profits, he worries that God will scold or "chide" him. And if God is anything like the lord from the parable, the speaker could get cast into a darkness even more fearful than the one created by his blindness.
"Doth God exact day-labour, light denied?" I fondly ask. […]
It has taken the speaker six lines to get through the part of the sentence that begins "When." Now he goes on to say what happens "when" he thinks about all the stuff he has described above. Namely, he wonders if God demands that people undertake hard, physical work, or "day-labour," when they don't have any light.
The speaker doesn't have any light because he's blind, but in Milton's metaphor he compares this condition to having to do work at night that you would normally do during the day – like, say, building a house or plowing a field.
The word "exact" means something like "charge," "claim," or "demand." You can "exact" a toll or a fee, for example. So the speaker wants to know if God demands work as a kind of payment that is due to Him.
The first section of the poem is completed by the words "I fondly ask." The word "fondly" means "foolishly," not "lovingly." The speaker accuses himself of being a idiot for even thinking this question.
Fortunately, "patience" steps in to prevent his foolishness. More on that in the next section.