Let's think about the verb ложиться/лечь. The first thing to notice is that it is one of only four verbs in Russian where the imperfective is reflexive and the perfective is not.¹ It conjugates like this:

The primary meaning of the verb is “to lie” in the sense of “to lie down.” The first thing to notice is that the thing you lie down on appears in the accusative case after the preposition на:

It's also possible to lie under something под + accusative:

We often find this verb in is in combination with спать. The resulting phrase means “to go to bed.”

¹ You get extra points if you can name the three other verbs. Lots of people can name two others, but if you get three others, you are special. Add a comment to this post to prove your prowess!

² The verb “to lie” in the sense of “to lie down” is currently shifting in English. The old standard for the past is to say “Yesterday I lay on the couch after lunch because my head hurt” and the standard for the infinitive can be seen in “My head hurts, and I need to lie down.” In modern American English now people often say “Yesterday I laid down on the couch after lunch” and “My head hurts, and I need to lay down.” This verb is one my personal pet peeves in life, and I'm going to defend the old literary norm either to my dying die or to the day my sister finally admits she has been saying it wrong all these years, whichever comes first.

Death seems to be a-comin’ awful fast.