为什么在过去时态中if后面加一般现在时

题目

为什么在过去时态中if后面加一般现在时
I realized that it doesn't matter if you don't understand every word
什么是客观情况?then i started to write my own original sentence using the grammar i was learning中文意思是什么

答案

因为从句中为“客观的情况 / 真理”
再举一例：
The geography teacher told us that the earth turns aroung the sun.