第一句话：When reports came into London Zoo that a wild puma had been spotted forty-five miles south of London, they were not taken seriously.
第二句话：It immediately ran away when she saw it.
大家对比这两句话的逗号，第一句话加了逗号第二句话没加逗号。当时间状语从句坐落于句首时就需要用逗号与主句隔开。语法中有这么一说吗?
关于你的问题，基本原则是：假如主句在状语从句之前，一般不需要逗号；假如主句在状语从句之后，一般要用逗号。
但这并非绝对的，实质语言是丰富多彩的，例外的状况也有不少，你举的例子来自新定义，下面我给你举几个词典的例句：
1. 状语从句在主句之前没用逗号的句子：
When I turned round he was pointing a dirty great gun at me. 我转过身来，他以一支老大的枪对准我。（牛津）
When he spoke I was expected to reciprocate with some remark of my own. 他在说话时，我也应该说点什么作为回话。（朗文）
2. 状语从句在主句之后却用了逗号的句子：
She made herself get better by sheer willpower, when everyone else had given up hope. 当所有人都已经不抱期望时，她仅凭毅力恢复了健康。（朗文）
She thanked her lucky stars that she had taken out insurance, when she was in an accident on holiday. 当她渡假出意料之外时，她庆幸自己办了保险。（剑桥）
Newton reasoned there must be a force such as gravity, when an apple fell on his head. 当苹果落在牛顿的头上时，他判断出这肯定是因为某种力的原故，譬如重力。（剑桥）
