Learn Russian: Он спохватился, оглянулся и быстро пошёл назад. - He suddenly realized something, looked back, and quickly went back.

Спохватиться is a very common conversational verb. It means to suddenly remember something you forgot, or to realize you made a sudden mistake. Here, it implies he suddenly realized he left something behind or was going the wrong way, prompting his sudden reaction.

This lesson is about the particle было, but it isn't in this sentence. How would adding it change the meaning?

If the sentence ended with он было пошёл назад, it would use the particle of thwarted action to mean he started to go back, but then something stopped him or he changed his mind again. Because было is absent here, the action was successfully executed: he actually went back.

Why use пошёл instead of шёл in this sequence?

Пошёл is the perfective form, indicating the start of a motion in one direction (he set off). Because this sentence describes a sequence of consecutive, completed actions (he realized, he looked back, he set off), the perfective aspect is required. Using the imperfective шёл would incorrectly imply he was already in the process of walking.

What is the difference between оглянулся and just saying посмотрел назад?

While both involve looking backward, оглянуться specifically means to physically turn your head or upper body to look behind you, often in a quick or sudden manner. It is the most natural and precise verb for this kind of physical reaction.