In English (and other languages) we have pairs of words that go together. If you use one, you should use the other. Some pairs that come to mind are:
not only—but also
on the one hand—on the other hand (some wags add “on the third hand”)
Greek has one called the “men—de” construction. Okay, here’s the Greek; I know you’re curious: μεν … δε.
See how funny it seems when someone gets it wrong? (Second cell.) The comic is Curtis, by Ray Billingsley.
Do any other pairs come to mind? Share in the comments.