What is the meaning and usage of 于之 and its contraction 焉 in classical Chinese?
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.