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