Learn Portuguese (Portugal): Às vezes, ele dá por si a cantarolar uma canção. - Sometimes, he suddenly realizes he is humming a song.

The structure dar por plus a reflexive pronoun (here, si) is an idiom that means to catch oneself or to suddenly realize one is doing something. It emphasizes the sudden awareness of an action you started without thinking.

Why do we need the preposition a before cantarolar?

The idiom dar por si connects to the action with a plus the infinitive (a cantarolar). This structure highlights that the action is already ongoing at the exact moment he notices it, functioning similarly to the European Portuguese continuous tense (estar a plus infinitive).

Why is the pronoun si used here instead of ele?

Since the subject (ele) is catching himself in the act, the pronoun must be reflexive. When a third-person reflexive pronoun follows a preposition (like por), it takes the stressed form si. If he were noticing someone else doing it, it would be dá por ele (he notices him).

What is the difference between cantar and cantarolar?

Cantarolar is a verb derived from cantar (to sing). The suffix indicates doing the action softly, repeatedly, or casually. It translates closely to humming or singing to oneself, which perfectly fits the absentminded nature of this sentence.