Stress
I have learned that, for example, the word "overflow", when stressed on the first syllable, it's a noun. While on the second syllable, a verb. Why it occurs? Is the way that English has used to make clear which word class that a certain word belongs? And stress is the most important thing that could hugely improve comprehension and speaking skills on English? What do you think about it?
Above all, thanks for the help.

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yes, you are quite correct, and it is a problem when learning. Children first learn by hearing and repeating, so it is natural for them. Adults who are learning English need to go through this process. consider researching the "shadow method" which is a system of listening and repeating a native speaker from a podcast or radio.Even students who have a good knowledge of English need to do this in order to practice stress and pronunciation.  you can see videoclips about it on my website www.teacherjoseph.com or you can google "shadow method, learning english" to find more information about it.

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Thank you Jerry. I see, when a foreign is learning my native language, I learn things that I haven't been taught before.

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That may be technically correct but I was never taught that as a native speaker.
No that I think about it, yes, that is correct. It comes so natural that we don't even think about it.

You're welcome. And yes, you're correct in your observation. It can be quite jarring sometimes.