Learn Italian: Mentre seguivamo questo sentiero, ho visto un grande lampo. - While we were following this path, I saw a large flash of lightning.

This is the classic way to talk about an interrupting event in Italian! The imperfetto (seguivamo) sets the background scene, which is an ongoing action in the past ('we were following'). The passato prossimo (ho visto) is the sudden, completed action ('I saw') that happened during that background event.

Why is mentre used here instead of quando?

Mentre means 'while' and specifically introduces an ongoing background action. It almost always pairs with the imperfetto tense. Quando means 'when' and usually introduces a sudden event. You could flip the structure of this sentence and use quando instead: Seguivamo questo sentiero, quando ho visto un grande lampo. (We were following this path, when I saw a big flash of lightning).

Why does grande come before the noun lampo?

Most adjectives in Italian go after the noun. However, a small group of very common descriptive adjectives, like grande (big), piccolo (small), bello (beautiful), and buono (good), often go before it. You could perfectly well say un lampo grande, but un grande lampo is very natural and common.

Why isn't questo shortened with an apostrophe here?