病気です。 means I have an illness. Not illness such as catching a cold. A bit serious one which you need to see a doctor with. 具合が悪いです。 means I'm in the bad condition as Jasmine says. We say that especially when having a headache/stomachache/diarrhea, feeling nauseous, or when you can't get away from your bed due to illness. 気分が悪いです。is used in 2 ways: when you feel nauseous and physically sick, and when something bad happens to you. Imagine when you ride the most terrible roller coaster several times. Your head would swim and you feel nauseous, wouldn't you? Then you would say 気分が悪いです。. And imagine when you see a crime scene with lots of blood, then you would feel worse or nauseous and say 気分が悪いです。. So you can say it when you see something gross or ghosts. One more example! Imagine when you get chewed out by your boss or teacher unreasoningly, then you would not get the reason,feel bad and complain 気分が悪いです。to your friend or family. We never say 病気です。and 具合が悪いです。when something bad happens to us. -Other ways to express depression or sadness- 辛い(つらい) Very useful expression when you're sad or having loads of work to do, a part of your body really hurts and so on. 病んでいる（やんでいる） A kind of slang, but useful and 2 ways to use. One is when you have a serious illness especially depression. Another is when you are depressed and very worried. It comes from a verb 病む（やむ）。 苦しい（くるしい）This is also useful and similar to 辛い. がっかりする　It means to be dissapointed and the same with 落ち込んでいる。 That's all we often say, and the words Jasmine says are also quite common.

病気です :be sick, be ill 具合が悪いです: be in bad condition 気分が悪いです: be sick, feel bad (We use 気分が悪い not only physically, we also use it mentally.) 風邪(かぜ)を敷く: catch a cold 機嫌(きげん)が悪い : be in a bad mood (only mentally） And as Jasmine said 悲しい、落ち込んでいる、ゆううつだ Hope it helps you. Sorry that my English isn't good.

You don't need "い" when you write 病気いです。 病気です is correct. Sometimes 気分が悪いmeans "getting angry" or "be annoyed". Differences are...an expression and tone his/her voice. がっかりする... be disappointed つらい...hard, painful, sad,difficult, tough

病気いです literally means "is illness" 具合が悪いです means something like "is in bad condition" 気分が悪いです is used when saying you feel sick or nauseous; so physical, not mental 悲しい　（かなしい）　means I'm sad. (direct translation: "sad") 落ち込んでいる　　（おちこんでいる） is a verb meaning to be depressed 憂鬱だ　（ゆううつだ）　is a less formal way to say you are blue/depressed