Use a d when there is a direct object.
Lie flat on the floor.
Beyond the present tense the pair can become more confusing because lay is the past tense of lie and laid is the.
Lay means to place something down flat while lie means to be in a flat position on a surface the key difference is that lay is transitive and requires an object to act upon and lie is intransitive describing something moving on its own or already in position.
I was laying the blanket on the floor.
The verb to lay means to place something in a particular position.
You ve been lying down all day.
If there is then use a form of lay.
Your best bet when deciding between the variations of lay and lie is to determine whether there is a direct object you re referring to.
A carpet or rug put a small pile of paperback books under your head to a comfortable height about 2 small paperback novels for many people point your knees at the ceiling so your feet are flat and your lower back is comfortable.
English grammar today a reference to written and spoken english grammar and usage cambridge dictionary.
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Because you need a direct object only with lay you will know that the past tense is laid.
The past tense of lie as in to tell an untruth is lied as you can see the past tense of lie is lay but the past tense of lay is laid which is a recipe for confusion to remember that laid as opposed to lain is the past tense of lay just memorize this phrase.
Lay or lie.