Learn Russian: Улитки часто ползают по этим мокрым камням. - Snails often crawl around on these wet stones.

The verb ползать is the multidirectional version of 'to crawl'. We use it here because the word часто (often) shows this is a repeated, habitual action. Furthermore, pairing a multidirectional verb with по implies moving aimlessly around the surface of the stones, whereas ползти would mean crawling in one specific direction.

Why do we use the preposition по with the dative case here (по этим мокрым камням)?

With verbs of motion, the preposition по plus the dative case is used to mean moving along, around, or all over a surface. It pairs perfectly with multidirectional verbs to show that the snails are covering the general area of the stones rather than making a single trip across them.

I sometimes mix up plural endings. Why is it мокрым and not мокрыми?

The word мокрым has the dative plural ending, which is required here by the preposition по. You might be thinking of мокрыми, which is the instrumental plural ending. A helpful trick is to remember that the instrumental plural almost always ends in -ми.

What is the nominative singular of камням? Did a letter disappear?