My husband says "you need an object for lay in the present tense. Example - you lay somthing down. lie is an intransitive verb (whatever that means ) that does not require a verb. The problem is that the past tense of lie is the same as the present tense of lay: Lie, lay, lain / lay, laid, laid "

so there you have it. I still get it all mixed up. I decided I would use the following to try to remember it - I lie down, but I lay the clay down.