by oneself

One should do one's homework by oneself.

Fish caught by oneself are delicious.

Wanting to become a wiseman by oneself is completely insane.

It's preferable to be by oneself than to be in the company of toxic individuals.

You'll stay and have tea with me, won't you? It's so lonesome to have tea all by oneself.

deDu bleibst doch noch zum Tee, oder? Es ist so einsam, ganz allein Tee zu trinken. — Pfirsichbaeumchen