It was nearly seven o'clock _when__he began to work.   这里为什么填的是 when? 怎么区别 it was...that 和 it was.when...how  do you suggest 是你怎么建议  那请问有没有 How do you suppose ,

that 引导无实际意义，when引导时间状语从句。
没有 How do you suppose.应该是 What do you suppose.......
do you suppose 和do you think 差不多，是用来做插入语的

简单，that无实意，只起连接作用，when引导的是时间状语从句

it was ...that ...是强调句,你可以把it was 和that 去掉,看剩下的部分是否是个完整的句子.若是完整句子,原句就是强调句
若这空填的是that,你把it was和that去掉就变成了nearly 7 o'clock he began to work.显然这个句子有语病,所以应该填when
如果你非要填that 就把句子改成it was at seven o'clock that he began to work.（加个at）
how do you suppose 是你怎么想的意思

根据意思的 “当”他开始工作的时候 已经将近七点了。 when是当……时候
it was when 引导时间状语从句
it was that 引导一般宾语从句 或者是强调句
第一个是你有什么建议
第二个 有这么个用法 是 你怎么认为