Learn French: La tension baisse après que les joueurs marquent un but. - The tension drops after the players score a goal.

While avant que (before) takes the subjunctive because the action is uncertain or hasn't happened yet, après que (after) takes the indicative. This is because the action in the après que clause is treated as a completed, actual fact. Even native French speakers sometimes mistakenly use the subjunctive here because it sounds similar to avant que, but the strict grammatical rule requires the indicative.

Could I just say après avoir marqué instead of après que les joueurs marquent?

Not in this exact sentence. You can only use après + past infinitive (like après avoir marqué) when the subject is the same in both halves of the sentence. Here, the subject of the first part is la tension, and the subject of the second part is les joueurs. Because the subjects are different, you must use après que + a conjugated verb.

Both baisse and marquent are in the present tense. Can après que be used without a past tense?

Yes. While you often see après que followed by a past tense when describing a specific sequence in the past, here both verbs are in the present because it describes a general truth or a habitual action: whenever they score a goal, the tension drops.

Why does the sentence use baisse for drops? Could I use tombe?