怎样选择判断用that而不能用when(强调句和定语从句)

问题描述:

怎样选择判断用that而不能用when(强调句和定语从句)
It was on the morning of May 1st when I met my friend at the airport.( when为什么错?)
It was on the morning of May 1st that I met my friend at the airport.

判断的依据是:将It was (is)...that...同时去掉,句中不缺省成分,则为强调句,否则有可能是定语从句或其他从句,要看具体问题进行分析.该题中,如果把It was (is)...和所填部分去掉就形成了:“ On the morning of May ...