It was about eight o'clock in the evening, I was watching TV quietly in my living-room when the phone rang.

quietly 是形容词 quiet（安静的）加上副词后缀 -ly 构成的副词形式，表示"安静地、轻声地"。 在这个句子中，quietly 修饰动词 watching，描述看电视时的状态是安静不吵闹的。