Data Modelling Issue

[Admirer A]

I am developing a job management site.It consists of a login page and a login table which stores username and password.This username is the primary key and is used to refer every other table in my database.So should I be using weak entity sets to define all other tables.Or should I define a weak entity set if and only if its truly necessary.And what should I do in this peculiar situation.
In general:I have a two strong entities STUDENT,EMPLOYER which are identified by username,password in the login table which makes both entites weak,I guess.Or what should I do in this peculiar case.Help me