Learn Swedish: Jag hör en pojke vissla medan han strosar i parken. - I hear a boy whistle while he strolls in the park.

Because höra (to hear) is a verb of perception. Just like in English ("I hear a boy whistle"), when you hear or see someone do something in Swedish, you use the object (en pojke) followed by the bare infinitive. This means you use the dictionary form of the verb without the word att (to).

In English, I'd probably say "I hear a boy whistling". Can I use the Swedish present participle visslande instead?

You can, but it sounds less natural. While English relies heavily on the "-ing" form for ongoing actions, Swedish strongly prefers using the bare infinitive (vissla) after verbs of perception like höra (to hear) and se (to see).

Since vissla is an infinitive, why is strosar in the present tense?

They belong to different parts of the sentence. Jag hör en pojke vissla is the main clause. The conjunction medan (while) starts a brand-new subordinate clause with its own subject (han). Because of this new subject, strosar must be conjugated in the present tense to show what he is actively doing.

Is there a difference in feeling between strosar and the more basic word går (walks)?