英文文法問題

1.supper 跟 dinner 一样吗?
dinner是比较正式的
2.They can have no certainty of success.
中间那个can为什麼要加进去呢?可以去掉吗?
可以,只是为了强调"不可以"才用can
3.No gods allowed.为什麼dog是复数呢?no 的后面不是单数吗?
可以用复数
4.I allowed that it had been a mistake.
中间那个had been 为什麼不是was呢?
强调已经过去的(是个错误),当然也可以用was.
5.还有下列3句的用法不太懂
(句子是从字典上抄来的,但我不知文法该怎麼用)(as的用法)
Old as I am,I can still fight.尽管像我这么年老了,我还可以战斗.
Try as you may,you will never succeed.像你这么尝试法,你永远都不会成功.
Much as I want to,I can't go.虽然我很想(去),但我不能去.表示像我这么想去.
as...像.如.