Ninety minutes, and then some
A match is two halves of forty-five minutes with a fifteen minute break between them. The clock runs continuously and is never stopped, not for injuries, not for substitutions, not for the ball going out of play.
Because time is lost anyway, the referee adds some back at the end of each half. You will see a board held up showing a number of additional minutes. That is a minimum, not a maximum, and the referee alone decides when it ends. This is why matches finish at unpredictable moments and why the phrase you will hear most often is that there is still time.
If you follow U.S. sportThere is no two-minute warning and no way to ice the clock. A team leading by one goal cannot kneel out the last three minutes; they have to keep playing.
Eleven players, and only three or so changes
Each team has eleven players on the pitch, one of whom is the goalkeeper and is the only player allowed to handle the ball, and then only inside their own penalty area.
Substitutions are strictly limited, usually to five per match made across three separate stoppages. A player who comes off cannot return. There are no separate offensive and defensive units: the eleven who start are largely the eleven who finish, which is why stamina and fitness are talked about so much.
If you follow U.S. sportClosest to soccer's substitution rule is hockey without line changes. Everyone plays both ways, all game.
A draw is a real result
Most league matches that finish level simply end level. There is no overtime and no shootout. A nil-nil draw is a legitimate outcome and both teams take something from it.
Knockout competitions are different, because somebody has to go through. Those go to extra time, thirty further minutes, and then a penalty shootout if still level.
If you follow U.S. sportThis is the single hardest adjustment for an American viewer. Roughly a quarter of league matches end in a draw, and that is by design rather than a failure to settle anything.