싶다 is the base form of those,싶은데,싶어서. I think you already knew that, but beside there are many different types like 싶으므로,싶지만,싶으면,etc. XP
Anyway 싶다 implies wish or thought of the verb locating before 싶다. ex) 가고+싶다, 먹고+싶다.
And your question, like he says ~ 데> but,~서> therefore,
자고 싶은데(but) 이불이 없다.(there is no blanket)
싶어서 meaning cause, then an effect comes after, 자고 싶어서>>누웠다.(lay down)
Normaly it works like that.
However, in that context 싶은데 means I wish I learnt~, so that It's good to help each other. like "therefore" you can just think of that there is one sentence disappeared between, 나는 한국어를 배우고 싶은데,(가르쳐 줄 사람이 없어요) 서로서로 도와주면 좋겠(어/네)요,
도와(주면/준다면/주겠다면/주신다면,etc..) 면 means "If", if we help each other, it'll be good, 좋겠(어/네)요.
좋다 is the base verb form of them, and derived from it on are these,좋겠(다/어/네) +
요 for politeness.
좋겠다 is used for when they wish somthing'll happen as you've thought, you'd be happy for it,
ex)시험에 합격하면 좋겠다.
좋겠어 is for rather you give your opinion
ex)난 너가 학교에 갔으면 좋겠어.>> spoken Korean
hum..they네,어,다 are used in the same way in general except only a few conditions,which you should learn later getting to understand more about Korean, XP cause difficult and too many to tell