Learn Russian: Собака, у которой болит нога, хочет полежать. - The dog, whose leg hurts, wants to lie down for a bit.

The prefix по- adds the meaning of doing the action for a short, limited time. While лежать just means to be in a lying position, полежать is a perfective verb meaning "to lie down for a bit." Since the dog's leg hurts, it wants a short rest.

Wait, doesn't лечь mean "to lie down"? What is the difference between лечь and полежать?

Лечь is the standard perfective verb for "to lie down," but it only focuses on the physical change of position (going from standing to lying). Полежать focuses on spending a little time in that state. The dog doesn't just want to hit the floor; it wants to rest in a lying position for a while.

How exactly does у которой translate to "whose"?

Russian expresses "whose" in a relative clause using the standard structure for possession: у + genitive. Because it refers back to собака (a feminine noun), we use the feminine genitive form of который (which/who). Literally, the clause means "The dog, at which the leg hurts..."

Why is the verb болит in the singular form?