Use this pattern whenever you want to find out about the timing of an event in the past, present, or future.

当你想知道过去、现在或者未来某件事发生的时间时，你都可以使用这个句型。

When 课堂 - SpeakingMax英语口语达人

You could use PCR to do that because if you amplify a gene in PCR, that gene had to be present from the beginning.

你可以用PCR技术来鉴定,因为如果你能用PCR扩增某一段基因,说明它从一开始就肯定是存在的

耶鲁公开课 - 生物医学工程探索课程节选

You can also use PCR to identify specific genes that are present in a biological sample.

你也可以利用PCR技术去识别,一个生物样本中的某段特定基因

耶鲁公开课 - 生物医学工程探索课程节选

And you will not use it for talking about stories or information in the present.

你也不会用它表示当前的情况与信息。

I was 课堂 - SpeakingMax英语口语达人

And you'll use "If I were" to talk about yourself in the present in relationship to the past.

你可以用“If I were”来比较现在的你和过去的你。

If I were 课堂 - SpeakingMax英语口语达人