既(jì) - both, as well as; since, given that in Mandarin

既 (jì) is almost never used alone. Its main job is the paired pattern 既...又... and 既...也..., which links two qualities that hold at the same time: 既聪明又努力, clever and hard-working both. Use 又 when the two items are the same kind of thing, and 也 when the second one shifts register or subject.

The negative version is 既不...也不..., which is how Mandarin says neither this nor that. Note that the second half needs its own 不 or 没: 既不喜欢跑步，也不喜欢游泳, not a single negation carried over from the first clause.

The other 既 you will meet is 既然, the causal conjunction meaning since or given that, and that is a separate word with a separate feel. The trap is assuming 既 on its own can carry that meaning in speech. It cannot outside fixed literary phrases like 既来之，则安之. If you want since, use 既然.