Learn Polish: Chłopiec zadrżał, bo było mu zimno. - The boy shuddered because he was cold.

The base imperfective verb is drżeć (to tremble or shiver). By adding the prefix za-, we create the perfective verb zadrżeć. Here, za- acts as an inchoative prefix, meaning it marks the sudden onset of a state or action. Instead of an ongoing "he was trembling," zadrżał means "he shuddered" or "he suddenly started to tremble."

Why do we say było mu zimno instead of "on był zimny"?

To describe how someone feels physically or emotionally (like being cold, hot, or sad), Polish uses an impersonal construction: a neuter verb (było) + the person experiencing the feeling in the Dative case (mu = to him) + an adverb (zimno). If you translated "he was cold" literally as "on był zimny," it would mean his actual body was cold to the touch (like a snowman or a corpse), not that he felt cold.

Why are we mixing a perfective verb (zadrżał) and an imperfective verb (było) in the same sentence?

This is a classic way to show a sudden event happening against a background context. The imperfective było mu zimno describes an ongoing background state (he was feeling cold over a period of time). The perfective zadrżał (he shuddered) is the sudden, completed action that happened as a result of that ongoing state.

Why is the pronoun mu placed between było and zimno? Could it go at the end?