"You are lying down already."
Translation:Te már fekszel.
I disagree, "(te) már lefekszel" is a correct translation, it does not necessarily mean that you're going to bed. It could also mean e.g. that we are on a hiking tour and you're getting tired after an hour and you lie down under the trees. Or you can lie down in a boxing ring, see "oda az álmod, ha lefekszel" ('your dream is over if you lie down' - Ilyen a boksz by LGT).
The verb is fekszik (third person singular). It is an -ik verb, which does not concern you in the current context, but it means that the base of the verb is "feksz". It is this that requires the second person ending applied to it. Since that ends in one of -sz, -s, -zs, -z, (namely -sz) then it gets one of -ol, -el, -öl, (namely -el) rather than -sz, -asz, -esz.
You've tried giving it two second person singular endings :)
The only other thing to know about -ik verbs is that the first person singular indefinite is fekszem (same as the definite).
It's not the most natural word order for this sentence, and there aren't many contexts where it would be used. It means something like: "now it's you who's lying". As if some people took turns in lying down. Or as if two people were wrestling, and one of them seemed stronger, but suddenly the other one knocked him to the ground: "You shouldn't be so arrogant anymore, már te fekszel."
Not really irregular. If the stem ends in a sibilant (-sz, -s, -z, -zs), then the harmonising -ol, -el, or -öl is used. The word, "fekszik", is an "-ik" verb, so its stem is "feksz". The other significance of "-ik" verbs is that the first-person singular indefinite conjugation uses the first-person singular definite conjugation.