Questions & Answers about Мой брат быстро успокаивается после ссоры.
The ending -ся (or -сь after a vowel) is the reflexive marker. It usually means:
- the subject does the action to themself, or
- the action happens by itself / to the subject, not to some other object.
Compare:
Он успокаивает ребёнка. – He calms the child (down).
→ non‑reflexive, he calms someone else.Он успокаивается. – He calms down.
→ reflexive, he himself becomes calm.
So успокаивается = “(he) calms down / becomes calm,” not “calms someone.”