1）
it is that 句型若强调的主词有二个
那 that 后面的动词要用单数动词还是复数？
看过二种
It is cars and factories that release a great deal of gas.
It's the clash of characters and plot that makes an interesting narrative.
2）
Only if vs Only when
我看到的only if例句会加逗号，only when 却没有，请问为什么？
Only if the government will take some appropriate measures, will this intractable problem be tackled.
Only when you are in adversity will you learn who are your true partners.
Thanks!