You don't have to..
However, what will you choose as your 1st year electives? (remembering that in both semesters, you do 3 core's and 1 elective which would form part of your major)
yes but dont worry if that elective you choose doesnt end up being your major. you are allowed to choose 2 (sometimes more) electives (ie: commerce subjects that arent part of your major) in your course
if you do comm/law
you do the 6 core commerce sujects and 2 law subjects in first year.
in second yr, you'll start choosing your electives for your major