メアリーは泳げないし、ジョンもまた泳げない。

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

(at the end of a phrase) notes one (of several) reasons

too; also; in addition; as well; (not) either (in a negative sentence); both A and B; A as well as B; neither A nor B (in a negative sentence); even; as much as; as many as; as far as; as long as; no less than; no fewer than; even if; even though; although; in spite of; further; more; again; another; the other

again; and; also; still (doing something)