1·In 'speak quietly', the adverb 'quietly' is a modifier.
在speak quietly中，副词quietly是修饰语。
—— 《牛津词典》
2·Knowing there was a camera around when my friends came to my home, they would speak quietly and carefully.
知道周围有摄像头，朋友来我家的时候会小心翼翼地小声说话。
3·He likes to speak quietly.
他喜欢低声说话。
4·We speak quietly and respectfully with one another.
交谈中，我们轻轻地说，并且心存敬重。
5·Please speak quietly, I think someone is listening in.
请悄悄地说，我觉得有人在偷听。
6·I said: "I understand you; I will try to speak quietly."
我说：“我理解你，我说话会尽量小声些。”
7·At table, you should try to speak quietly and smile a lot.
吃饭时，你应该尽量轻声谈话，并尽可能微笑。
8·We cannot learn from one another until we stop shouting at one another – until we speak quietly enough so that our words can be heard as well as our voices.
除非我们停止漫骂对方，将音量降下来，让对方能听清我们的言辞，否则我们不可能从他人身上学到什么。
9·In the school hallways, the students are supposed to speak and act quietly.
学生在学校走廊里说话和活动时应降低音量、放轻动作。
10·Force yourself to react more slowly and quietly than usual; angry people have a tendency to speak louder and faster.
强迫你自己强迫你自己去慢点、礼貌点去应对；愤怒的人有倾向讲越来越响越来越快。