This一般用于近指,一般是补充上一句的句子,或者说明一种情况和一种事物,就用This,This是These的单数形式,有时This和The比较难选择.
eg:Air flight would delay due to the bad weather,this is commonly seen.
That一般用于远指,定语从句,状语从句等等从句需要用That来联接.
eg:We have a hope that people landed in Malaysia Airlines Flight MH370 still be alive and will come back safely
It一般指动物罗,有时同位语从句一般用It开头
eg:It is a faith/wish/belief /(the way) that.