Learn Russian: Грузовик едва не облил меня грязью.

Почти is generally used for states, amounts, or actions that actually progressed to a certain degree. Едва не (literally "barely not") is used specifically for sudden, often negative actions that were completely averted at the very last second. The truck didn't splash you at all, but the event was dangerously close to happening.

Why is the verb облил in the perfective past, considering the action never actually happened?

When using едва не or чуть не to describe a sudden "near miss," Russian grammar requires the perfective past tense. Even though the action was averted, you are expressing that a single, momentary event almost reached its full completion.

Why is the word for "mud" (грязь) in the instrumental case (грязью)?

The verb облить (to drench / to pour over) takes the accusative case for the person or object being splashed (меня) and the instrumental case for the substance being used. Therefore, облил меня грязью literally means "poured over me with mud."

How does the prefix об- change the meaning of the verb here?

The base verb is лить (to pour). The prefix об- often adds the meaning of covering an object's surface completely or going all around it. Combined, облить means to pour liquid all over someone or something, which translates perfectly here as "to splash" or "to drench."