为什么I‘m typing the letter 前要用am?实义动词前不用是要用do,does吗

为什么I‘m typing the letter 前要用am?实义动词前不用是要用do,does吗
While he was writeing on the blackboard the children were talking,在这句中writeing 是实义动词吧,为什么前面会加was,talking前加were
如果去掉was 和were 改成while he writeing on the blackboard ,the children talking,为什么不对？
为什么But this morning he got up at 6 o'clock,中got up 前不用加was.同样是动词为什么writening 与talking前要加was和were而got前就不加was