Question

here come`s a bus

它的主语是谁，或者说here是不是主语?

Answer 1

它的主语是a bus。here不是主语，这是完全倒装句。完全倒装有以下几种形式：
当表示地点的状语位于句首时，其后用完全倒装形式。这类倒装句的谓语通常是动词be和come、go等表示移动或动态的不及物动词。如：
Here’s Robbie.
There’s Richard.
Here comes the bus.
There goes the bell.
There goes the last train.
Over the wall came a shower of stones.
Among these people was his friend Jim.
By the window sat a young man, magazine in hand.
Round and round flew the plane.
Out was the nail taken.
Off came the wheel.
表语位于句首时，后面要用完全倒装形式，谓语为联系动词，如：
Happy is the country that has no war.
Faint grew the sound of the bell.
有时为了强调，可以把进行时里的现在分词、被动语态里的过去分词，或作表语的不定式位于句首，构成完全倒装，如：
Standing in the doorway was a man with a gun.
Suspended on the ceiling is a beautiful light.
To be considered carefully are the following questions.
希望我能帮助你解疑释惑。

Answer 2

here 不是主语，这是一个倒装句，主语是 bus