Learn French: Elle est contente que tu t'abonnes à la chaîne. - She is happy that you are subscribing to the channel.

Because they follow the exact same rule! Any expression of emotion or feeling—like aimer que (to like that), être triste que (to be sad that), or être content que (to be happy that)—requires the following verb to be in the subjunctive mood.

Is t'abonnes really in the subjunctive here? It looks exactly like the normal present tense.

Yes, it is! For regular -er verbs like s'abonner, the subjunctive forms for je, tu, il/elle/on, and ils/elles are spelled and pronounced exactly the same as the regular present tense. You only see a visible difference in the nous and vous forms, which add an extra i (for example, que vous vous abonniez).

Why is it tu t'abonnes instead of just tu abonnes?

The French verb for 'to subscribe' is reflexive: s'abonner. Literally, you are subscribing yourself to something. Because the subject is tu, the reflexive pronoun is te, which shortens to t' before the vowel.

Do we always need the preposition à after s'abonner?