he walk over to see what happened...什么时候用what is happen 什么时候

语法问题
1、he walk over to see what happened...中的walk不能用动词原形,可以改成walks或walked
2、如果是要表达“去看正在发生什么”,后面就用to see what is happening；若是要表达“去看发生了什么”,后面就用to see what happened