Learn Russian: Дедушка тихо постукивает тростью.

This combination of prefix and suffix adds two specific nuances: intermittent action (doing it on and off) and attenuation (doing it lightly). Instead of a continuous or heavy knocking (стучать), постукивает means he is tapping the cane lightly, repeatedly, and casually over a period of time.

Is постукивает imperfective or perfective? The prefix по- usually makes verbs perfective.

It is imperfective. While по- often forms perfectives, the suffix -ива- is a strong marker for the imperfective aspect. When used together in this intermittent-attenuative pattern, they create a brand new imperfective verb that describes an ongoing but low-intensity, repetitive action.

Why is тростью in the instrumental case?

The word трость (cane) is a third-declension feminine noun, so its instrumental ending is -ью. It is in the instrumental case here because it acts as the physical tool used to perform the action. The grandfather is tapping with his cane.

Why does the English translation say lightly when тихо normally translates to quietly?

In Russian, sensory concepts like low volume and low physical force often overlap. When paired with a verb of physical impact like постукивает, doing it тихо means producing very little noise, which naturally implies a light, gentle physical touch.