The two sentences you presented are examples of the rhetorical device in English known as contrast and comparison, which emphasizes the difference or similarity between two or more things or ideas. As a rule, such contrast and comparison statements take the form of compound sentences that contrast or compare ideas in two coequal and coordinate clauses.
Let’s examine the first sentence:
“It is easy to kill a bird on the wing that flies straight, not so one that twists and turns.”
This sentence uses the contrastive phrase “not so”—a negation of the premise in the first clause—to emphasize the difference between the ideas in these two sentences: “It is easy to kill a bird on the wing that flies straight” (a positive statement) and “It is not easy to kill a bird that twists and turns” (a negative statement). The contrastive phrase “not so,” which has the same meaning as the conjunction “but not,” fuses the positive statement and the negative statement into a single compound sentence, with the repeated clause “it is easy to kill a bird” used only once for brevity. In other words, the contrastive phrase “not so” eliminates the need to repeat that clause for the idea in the second sentence, thus streamlining the combined sentence as well as making it more forceful and persuasive.
The word “one” that follows “not so” in that combined sentence is, of course, the indefinite pronoun that’s used to indicate a certain indefinitely indicated person or thing. Here, it’s being used instead of the noun “bird.” Of course, this replacement of a noun by a pronoun makes it possible to avoid the tedium of repeating the same word in the same sentence.
Now let’s examine the second sentence:
“The gamester never plays the card the opponent expects, still less the one he wants.”
This time we have a sentence that uses the comparative phrase “still less” to emphasize the even smaller likelihood of a second outcome happening compared to that of the first: “The gamester never plays the card the opponent wants” (the second sentence, which is a negative statement) and “The gamester never plays the card the opponent expects” (the first sentence, also a negative statement). The comparative phrase “still less” fuses the two negative statements into a single compound sentence that uses the clause “the gamester never plays the card” only once to make the point. Here, the comparative phrase “still less” eliminates the need to repeat that clause for the idea in the second sentence, thus streamlining the combined sentence as well as making it more forceful and persuasive.
As in the case of the first sentence, the word “one” that follows “still less” in that combined sentence is the indefinite pronoun for a certain indefinitely indicated person or thing. Here, “one” is being used instead of the noun “card” in the first clause.
The important thing to remember in comparison and contrast sentences is that they are meant to give readers or listeners a clear yardstick for appreciating similarities or differences between two things or ideas—the better to persuade them to accept the idea that the comparison or contrast is meant to support. In the case of the contrastive and comparative sentences we dissected here, they provide comparisons in the form of pairs of specific alternative outcomes.
