How Can I Concatenate Column A and Column B into Column C in Excel?

Column A has first name. Column B has last name. I need column C to be "display name" (first name and last name separated by a space). I assume this involves the concatenate function.

Bonus Question: Column D is email address. I need column E to be "username" (everything in email address left of the @ symbol). I assume this involves some regular expression function.

For the first part, yes you need CONCATENATE, for example:

=CONCATENATE(A1, " ", B1)

For the second, something like:

=MID(D1,1,FIND("@",D1)-1)

will give you everything up to the @ of an e-mail address.

Since variety spices life, here are some other options. Note that the ampersand "&" is the concatenation operator.