Learn German: Die Firma ist zu klein, um ihn einzustellen. - The company is too small to hire him.

This pattern translates to "too [adjective] to [do something]". In the first part, zu klein means "too small". The second part, um ihn einzustellen, means "to hire him" (literally: in order to hire him). Together, they express that the company's small size is the exact obstacle preventing them from hiring him.

Why is zu tucked into the middle of the word einzustellen?

The verb einstellen (to hire) is a separable verb, made of the prefix ein- and the root stellen. When an infinitive clause requires zu (like the um ... zu structure here), the zu gets sandwiched directly between the separable prefix and the root verb, forming one single word: einzustellen.

Is the comma before um mandatory?

Yes. In German, when you have an infinitive clause introduced by a conjunction like um, you must separate it from the main clause with a comma. This helps organize the sentence, since the action verb (einzustellen) is pushed all the way to the very end.

Why does ihn come right after um?

ihn (him) is the direct object of the verb einstellen, so it takes the accusative case. In an um ... zu clause, the word um comes first, the verb with zu goes to the very end, and all other information—like the object ihn—is placed directly between them.