Why is it only correct with the 就 and wrong without it? And should it be correct with either 如果明天下雨... or 如果下雨明天...?
I found out that 就 means "just",so I don't think it is not that important word.
It has many meanings. Another one is "then", which goes perfectly with "if". In an English if...then construction you can omit the word "then". The same might or might not be the case in Chinese?
"出門"sounds better than "出去"
Exactly. "出門" emphasizes "leaving a place", whereas "出去" is close to "to leave"
What about 离开? How do the three differ?