Descartes' Meditations: 1 & 2

I think you misunderstood Descartes Meditations. He was sort of after a method of determining how he can know something with 100% confidence. His argument for dreaming is that you can not know that this world is real because you can't prove that it is. It's sort of like a dream, you very well know that dreams are not real but what if the world was a dream that you never wake up from?

above all, to answer your question it is to determine what source of knowledge or what is the "thing" that can not be doubted.

i believe in meditation 4 he does come to proove the existance of things in the outside world.
Also, how do you know if your dreams are your dreams, that is, how do you know that your dream is not your reality..? and vice versa..

The point of any thought experiment (Descartes' included) is to explore what is possibly true, not necessarily to propose what is certainly true.

He proposes the idea of a deceiving demon because it is possibly true (based on the principle that we do not have the power to assess whether what we are experiencing is objectively true or false). The only thought which he concluded was necessarily true was his infamous line, "I think therefore I am" (if memory serves).

I am not sure just what you have in mind by those two arguments. But if you mean that argument that since, for all we know, we might be dreaming, or that for all we know, we might be being deceived an Evil Demon, that none of our beliefs justified by sense-perception that we know anything are adequately justified, so that any belief we have that is justified by sense-perception can be known. The upshot is that if we believe that we know anything justified by sense-perception, that belief that we know anything based on sense-perception is false. There are several crucial assumptions here, but one of them is that any belief that we know must be adequately. justified. Another is that no belief that we know that is based on . sense-perception is adequately justified. Still another crucial assumption is that unless our justification for a belief is such that it guarantees the truth of the belief, such justification is inadequate. And this entails that only deductive justification is adequate justification. I don't know what crucial assumption your teacher has in mind, but you would know if you attended and listened in class.