You may learn some complicated and simple words in free Chinese lesson. But the important thing is that you can explain the complicated words by simple ones. In 空调在冬天使用频率低, 频率 is a difficult word for you to understand, it means the frequency we use something. So we can replace it with次数, meaning the times we use things. After reading the sentence with 借鉴经验 in free Chinese lesson, you should know 借鉴 indicates to learn something good from others. So we can explain with 吸取, implying to absorb.

You may meet the sentence with 乃至 like  循环系统的衰老是导致生命衰老乃至死亡的主因 when you study Mandarin online. You should know it is similar to “even” and followed by a further degree with. Therefore, 甚至 is a proper word as a replacement. The word 意外 is often seen in free Chinese lesson in the sentence like 我被这意外的惊喜诱惑得失去了理智. You should know it is an adjective here to describe something unexpected. We usually say 想不到 easily to express the same meaning.