Learn Spanish (Latin America): De repente, el payaso se puso a silbar.

While poner does mean "to put", the reflexive phrase ponerse a followed by an infinitive (like silbar) is a special expression. It means "to start" or "to set about" doing an action. It often implies starting something suddenly or spontaneously.

Could I just say empezó a silbar instead?

Yes! Empezó a silbar or comenzó a silbar are perfectly correct and mean the same thing. However, native speakers use se puso a very often in everyday conversation, especially when the action feels sudden, unexpected, or energetic, which fits perfectly with a clown.

Why is it se puso (preterite) instead of se ponía (imperfect)?

The phrase De repente (suddenly) is a huge clue. It tells us that the action happened abruptly at a specific point in time, rather than being an ongoing background habit. Because he started whistling at a specific, sudden moment, we need the preterite form se puso.

Do I always need the word a before the verb?

Yes. When you use ponerse to mean "to start doing something", it must be linked to the following infinitive verb using the preposition a. Without it, the sentence would be grammatically incorrect.