know : 普通用词，多指通过学习、经验或他人传播而得到知识，含直接知道的意味。
- She knows the name of every kid in the school.
- I don't know anything about this.
- Even small amounts of these substances are known to cause skin problems.
- The authorities know him to be (= know that he is) a cocaine dealer.
learn : 通常指通过他人而获得消息或情况，侧重从不知到知的变化过程。
- We were all shocked to learn of his death.
- I later learned (that) the message had never arrived.
- I only learned about the accident later.
comprehend : 侧重熟悉了解的过程。
- I fail to comprehend their attitude.
- He doesn't seem to comprehend the scale of the problem
- I'll never comprehend why she did what she did.
- I don't think he fully comprehends that she won't be here to help him.
understand : 指对事物已有彻底的认识，不仅知其性质、含义和细节，而且了解其内外的关系。
- She explained the whole idea again, but I still didn't understand.
- Is there anyone here who understands Arabic?
- We still don't fully understand how the brain works.