Postby Shaydwyrm » October 21st, 2007 8:48 am
First of all, it's つかれています(tsukarete imasu), with no long "i" sound. And yes, the casual form is つかれている(tsukarete iru), which is often contracted in spoken language to just つかれてる(tsukareteru).
As for your other question, the simple answer is that that's just the way it is. つかれてです "tsukarete desu" is simply not grammatically correct, just like "I tired" or "I is tired" is not grammatically correct. つかれる "tsukareru" is a verb, so it functions in a fundamentally different way than an adjective like さむい "samui" or a noun.
You may be confusing the animate existence verb いる "iru" with the auxiliary verb いる "iru", which sound the same but are used in different places. つかれている "tsukarete iru" is not saying "tiredness exists", but rather something along the lines of "the state of becoming tired continues". I'm bad at glossing things into English, but maybe you get the idea?