Problem about how to use "lay down" in a sentence.

[Suthipong]

What does it mean by the phrase "I lie down"?
Can we say only a word "lie" without "down"?
I usually lie on sofa in the afternoon.(or lie down)?
I was lying on sofa and almost gone asleep when you called me. (or I was lying down on sofa)?
I lay on sofa at 2 o'clock yesterday. (or I lay down)?

 

[SlickVic9000]

(Not a Teacher)

With the exception of the 3rd sentence, you could use either one. Please note that you may hear native speakers say 'lay down' as well.
This is because 'lay' and 'lie' conjugate almost exactly the same, often resulting in their meanings being confused.

 

[Rover_KE]

You lie or lie down on the sofa.

'I was lying/lying down on the sofa and had almost gone to sleep/fallen asleep when you called me.'

Rover​