Learn Russian: Пока собака спала, кот лез в узкую трубу. - While the dog was sleeping, the cat was crawling into a narrow pipe.

The sentence focuses on a single, continuous action happening in one specific direction (into the pipe) at a specific moment in time (while the dog slept). Лез comes from the unidirectional verb лезти. If you used лазил, it would imply the cat was repeatedly crawling in and out of the pipe, or aimlessly exploring around it.

I thought лезти meant to climb. How does it mean to crawl here?

Лезти (and its multidirectional counterpart лазить) doesn't just mean climbing up a tree or a ladder. It is the go-to Russian verb for squeezing into tight spaces, getting into things you shouldn't, or moving awkwardly on all fours. Because a pipe is an enclosed space, crawling or squeezing is the most natural English translation.

Why are both спала and лез imperfective verbs?

They describe simultaneous actions in progress. Using two imperfective past tense verbs joined by пока (while) shows that the dog's sleeping and the cat's crawling were happening at the exact same time as a continuous background scene. If the cat had completely finished getting into the pipe while the dog slept, you would use the perfective verb залез.

Why is it в узкую трубу (accusative) rather than в узкой трубе (prepositional)?