すでに（既に）
Already (written, formal)machine translation
Formationすでに + verb (usually in the た-form or ている-form)
すでに means "already" — the written, formal version of the もう you've been using since N5: 会議はすでに始まっている (the meeting has already started). The meaning doesn't change, only the register — news reports, announcements, and business email favor すでに; everyday conversation sticks with もう.
Because it expresses something ALREADY completed or a state already in place, すでに almost always goes with the た-form or ている-form: すでに売り切れた (already sold out), すでに知られている (already known). It also often stresses the feeling of "too late": you arrive and the train すでに出発していた.
Note the opposite direction: "not yet" in writing is まだ〜ていない or, more formally, いまだに〜ない — すでに is not used in negative sentences of that kind. When a reading question asks you to choose between もう and すでに, look at the register of the whole passage.
Examples
会議はすでに始まっています。
The meeting has already started.
そのチケットはすでに売り切れました。
Those tickets have already sold out.
駅に着いたとき、電車はすでに出発していました。
When I got to the station, the train had already departed.