it is not yet eight o'clock when he arrived home.什么从句?

it is not yet eight o'clock when he arrived home.是状语从句,不过不是it is 而应该是It was...
it is 1999 when i met him是状语从句,同样is 应该改为was.再如：It was nine o'clock when I got there.我到那里时时间是9点.
it is in 1999 that i met him是强调句型.同样is应改为was.
那it is in 1999 when i met him 就不可以吗,不能理解为时间状语从句吗?when i met him,it is in 1999.
不可以.因为你如果想用状语从句,那就应该是It was 1999 when I met him.
而in 1999则是在强调这个介词短语所充当的时间状语.
强调句句式是固定的：It is/was＋被强调部分＋that（人/物）/who/whom(人）+非被强调部分
如：I met them in the street yesterday.
It was I that/who met them in the street yesterday.强调主语
It was them that/whom I met in the street yesterday.强调宾语
It was in the street that I met them yesterday.强调地点状语
It was yesterday that I met them in the street.强调时间状语