A key sentence should be clearly written. While it may not be obviously true, it should be obvious what would be the case if it were true. It should be as free as possible of qualifiers that soften its point and therefore make it true in any case. A key sentence should have a relatively narrow range of interpretations at the outset, and the rest of the paragraph should support the claim it makes in precisely that sense. Perhaps most importantly, the meaning of the key sentence should be clear to the writer. It should already make sense the night before the paragraph is written, and it should gain in clarity as the writing proceeds the next day. Indeed, it is perfectly normal to rewrite the key sentence a few times while writing the paragraph. Writing a paragraph should clarify the meaning of its key sentence.
The key sentence of a paragraph should also be interesting. Keep in mind that it expresses roughly one fortieth of your argument in a standard journal article. If you were to list the forty key sentences in a paper separately without their supporting paragraphs (and you do well to do this every now and then), you should not be left with a list of trivialities. Some of the claims, of course, may only be interesting in the context of the other claims. Indeed, some of them may depend on the others for their meaning, and even their clarity. But in this context it should be clear why the reader needs to be told these things. It should be clear why the reader should be interested.
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In other news, Profacero reminds us that learning anything is like learning a second language. It is a "myth that only sports, music practice, and foreign languages need daily work". I completely agree with this. Whatever we become good at we become good at by concerted, daily effort, in a word, through discipline. In the case of becoming a scholar, the relevant discipline is simple. Write paragraphs on a daily basis about something you know for people who are knowledgeable on the subject. If you are an undergraduate and think you might want to become scholar, start now. Don't let anyone tell you scholarship is different, that it is best accomplished by binging and fasting. That's a myth.