I've never managed to get the hang of the difference between 'lay' and 'lie' as present tense verbs (and 'lay', 'laid', 'was laying' and 'was lying' as past tense verbs, for that matter). I know one is a transitive verb and the other is an intransitive verb, as I asked this very question of 63336 and that's the answer I got. However, that doesn't really answer my question because it doesn't help me understand which to use in which situation.
Can anyone explain this to me in very simplistic terms? I normally have no problem with grammatical issues so this is driving me bananas!