So means to such an extent. So is often used when we are talking about a very high degree of some quality. So can be used in most situations where very is also possible.
- Why are you so unkind?
- He is so stupid.
So is not used with adjective + noun.
- She is such a beautiful girl. (NOT She is a so beautiful girl.)
So can be followed directly by that-clauses. Structures with very are not possible in this case.
- He was so ill that we had to call a doctor. (NOT He was very ill that we had to call a doctor.)
So far means as far as this.
- Now that we have come so far, we may as well go all the way. (= Now that we have come as far as this, we may as well go all the way.)
So long as means on condition that
- You may borrow the book, so long as you keep it clean.
So far from means instead of
- So far from helping us, he hindered our progress. (= Instead of helping us, he hindered our progress.)
So as to means in order to.
- I will hurry, so as to not delay you.
So can be used instead of a word or phrase.
- I told you so! (= That is what I told you.)
In constructions like the following so can mean also.
- He is learning English and so am I. (Note the inverted word order after so.)
So can also mean therefore.
- She asked me to go, so I went.