Learn Italian: Osservavo l'uomo che sorseggiava il suo tè. - I was observing the man sipping his tea.

Both translations are technically correct, but this is a classic pseudo-relative clause. After verbs of perception like osservare (to observe), Italian frequently uses che plus the imperfect tense to describe an ongoing action. In English, we usually express this with an '-ing' word ('the man sipping'), but Italian prefers this che structure.

If 'sipping' is an '-ing' word, why can't I use the Italian gerund sorseggiando here?

In Italian, a gerund almost always refers back to the subject of the main verb. If you said Osservavo l'uomo sorseggiando il suo tè, it would mean you were the one sipping tea while observing him! To describe what the object (the man) is doing, you must use the che + imperfect structure, or an infinitive.

Can I use the infinitive sorseggiare instead of che sorseggiava?

Yes! With verbs of perception, you can also use the infinitive. Osservavo l'uomo sorseggiare il suo tè is completely correct and means the same thing. The che + imperfect form simply adds a bit more emphasis to the continuous, ongoing nature of the action in the background.

Why is the main verb osservavo in the imperfect tense rather than the passato prossimo?