When I finished it the night was half gone and there was no time left to compose a proper, respectable essay for Mr. Fleagle. There was no choice next morning but to turn in my tale of the Belleville supper. Two days passed before Mr. Fleagle returned the graded papers, and he returned everyone's but mine. I was preparing myself for a command to report to Mr. Fleagle immediately after school for discipline when I saw him lift my paper from his desk and knock for the class's attention.

老师：您好！

这段原文用的都是过去时态，所以我不明白为何在最后一句中用的是动词原 knock，我觉得有点疑惑。是否是因为“看见某人做某事”是 see sb. do sth. 所以，这里用的是动词原形 knock? 因为 knock 前面有个 and，是并列关系的缘故？

我这样理解正确吗？

烦请老师指导。多谢。