Her first impression was that English was difficult and people spoke too quickly, but soon she found that if she concentrates on the ideas, not the single words, she could understand!

(1)句中的 that 引导宾语从句，if引导条件状语从句。

按理说，if引导的条件状语从句应该放在句末的，但在本句中被移到了宾语从句中间。原句也可写成：

Her first impression was that English was difficult and people spoke too quickly, but soon she found that she could understand if she concentrates on the ideas, not the single words!

(2)类似的例句如：

I knew that if I made any sound, they would find me. 我知道我一发出声音，他们就会发现我。

He said that if we started at dawn, we would be there by noon. 他说，我们要是黎明出发，中午就能到。

(3)这类在高考题中经常出现的，所以需重视：

I tell my mom that if we’re forced to eat things, we may become ill. 我告诉我妈妈，如果我们被迫吃东西，我们可能会生病的。(2020全国卷Ⅲ改错)

He said that if he quit the job, he would lose his drive to work and succeed. 他说如果辞职，就会失去工作和成功的动力。(2018浙江卷完形)

Her mother told her that if she did something to change the problem that made her sad, she wouldn’t feel as bad. 她的母亲告诉她，如果她做一些事情去改变这个令她难过的问题，她就不会感觉那么糟糕了。(2017北京卷完形)