What do you mean by “日本”in English?What's the meaning of the word?（这里的mean为什么这样用）

What do you mean by “日本”in English?What's the meaning of the word?（这里的mean为什么这样用）
There will be a friend coming here to see me tomorrow?(为什么是comeing,will后面不是原型吗）
There were a large number of people collecting garbage.（为什么用ing）
I didn't hear the phone.I must be out.(为什么不用must have gone)
The hat which he is wearing is black.（可不可以用that,为什么)
He is the boy that bought a bike here.(为什么不用whom）
The man whom/who/that you saw just now is my English teacher.(为什么可以用whom/who/that 晕死）
I‘d like to talk with the man sitting next to me.（为什么ing?)