In casual speech, it is either replaced by だ or sometimes dropped all together. It really depends on who you are talking to and what you are trying to say. The best way to get a feel for this is to read as many sentences as possible, or listen to as much audio as possible. My textbook drilled certain rules into me, but I really didn't understand those rules until I had more contextual knowledge (if that makes any sense). Hope this helps, Stevie.