<p>SparkNotes, or another good outline, is a great resource. If you want to reduce your reliance on outlines, one way to attack confusing sentences is to figure out the structure, cut the extra words, and then piece it back together. For example, with the sentence you gave:</p>
<p>
[quote]
"Old people did not know enough once, perchance, to fetch fresh fuel to keep the fire a-going; new people put a little dry wood under a pot, and are whirled round the globe with the speed of birds, in a way to kill old people, as the phrase is."
[/quote]
</p>
<p>1) Structure: There are two main parts to the sentence, separated by a semicolon. The first part starts with "old people" and the second starts with "new people". So Thoreau is probably going to try to contrast two different personalities/behaviors.</p>
<p>2) Cut the extra words: in this sentence, things like "perchance", "as the phrase is", "with the speed of birds", "in a way". Cut it down to the basic, bare bones of the sentence - once you understand that part, you can add the other words back in to clarify the author's meaning further. So now your two parts look like this:</p>
<p>
[quote]
"Old people did not know enough once to fetch fresh fuel to keep the fire a-going;
new people put a little dry wood under a pot, and are whirled round the globe, to kill old people."
[/quote]
</p>
<p>3) So what does this mean? It'd be easier to interpret in context, but here's a shot. The "old people" start a fire, but don't know enough to add fuel to keep it going. The "new people" add fuel, which propels them forward. By now you should definitely get that the fire is not literal, if you didn't before. If someone starts something but can't sustain it, it will fail. You need to figure out how to add new fuel, so to speak. Thoreau is basically talking about new ideas and innovation, and how they keep society moving.</p>
<p>4) Now you basically understand the sentence, but there are still some confusing parts - like, what's this about killing old people? You can start adding back the words you removed in order to clarify these details. When you put back in the "in a way" and the "as the phrase is", it's clear that Thoreau doesn't mean literal killing - he means that the innovative, original new people will supplant the old people.</p>
<p>Does that help?</p>