What is the meaning and usage of 于之 and its contraction 焉 in classical Chinese? I've read 于 was a preposition, but there were many unrelated translations for it. 之 is as far as I know an object pronoun, like for instance him, her, them but can also mean somthing different.

How is this expression actually used in classical Chinese? What is its verbatim meaning? Why does the contraction 焉 sound so much different than the two characters?
My teacher told me that 焉 equals to 于(at) + 之(a kind of pronoun).

与 usually equals to "and".

But in 与君共饮, the 与 means do sth with sb or do sth together. 

陈涉少时，尝与人佣耕： (work) for.

吾孰与徐公美： Compare with someone/sth else.

朝过夕改，君子与之： Agree with ...

I haven't all of the uses of 与 because these are common. It's not good for you to have a study about all the uses of one word though some of them are not common because they will confuse you.

I hope these will help you.