Why do we have such complicated word usage rules in English? It's almost as if language is an evolving, organic form of expression.
Lie, Lay, Laid, Lain are some of the fiddliest words to get right. Almost as fiddly as your/you're apparently. So here's a quick reference.
In the present tense:
You (the subject) LAY things(objects) down.
'I lay my hands on the table top'
You LIE yourself down.
'We lie our backs down in agony'
Past tense:
You LAID something down.
'We laid the table together'
You LAY yourself down.
'I lay on the bed and hoped for sleep'
Perfect:
You HAVE/HAD LAID an object.
'I had laid down my pen'
You HAVE/HAD LAIN yourself down.
'They have lain down and offered themselves to the evil God of grammar'