there stands a bookstore

there,now,here在这里是副词,他们在句首,并且主语是非人称代词时,一般需要倒装.是为了表示强调.
这三个句子的主语其实分别是：a bookstore,twenty books,your turn.所以动词分别是：stands,happens,comes.
正常语序应该是：
A bookstire stands there.
Twenty books happens to be left there.
Your turn comes now.
The bus comes here.