I thought this was a gentle exposition of the difference between “lay” and “lie”. Basically,

The main difference between the two words is that lay is a transitive verb, while lie is an intransitive verb.

and

Verb Infinitive Past Tense Past Participle
lie lie lay lain
lay lay laid laid

The summary is the best part, emphasizing the lesson.

So here’s the drill:
You need to lie down today, yesterday you lay down, in the [ast you have lain down.
Today, you lay the book on the table. Yesterday, you laid the book on the table. In the past, you have laid the book on the table.