Learn Russian: Я чуть не наступил на жука, который медленно полз по тротуару. - I almost stepped on a beetle that was slowly crawling along the sidewalk.

The sentence describes a specific moment in the past—the exact instant you almost stepped on it. At that moment, the beetle was in the middle of a continuous journey in one general direction. For an ongoing action at a specific point in time (often matching the English past continuous "was crawling"), Russian uses the unidirectional verb полз.

Why do we use чуть не here instead of почти for "almost"?

Чуть не is used with past perfective verbs (like наступил) to express that an action narrowly failed to happen, especially an accidental or undesirable one. If you used почти, it would sound like you were actively trying to step on the beetle and completed 90% of the task!

Why is it на жука? Is жук in the genitive case here?

It is actually in the accusative case! The verb наступить (to step) takes the preposition на plus the accusative case to show the target of the step. Because a beetle is animate (a living creature), its accusative form matches its genitive form, so жук becomes жука.

Why does по take the dative case (тротуару) in this sentence?