"Don't native English speakers know more than 70 percent of English dictionary usually? Or they learn them in their schools or university?"

Absolutely not. Estimates of the vocabulary size of native speakers run in the 10,000 to 40,000-word range. According to

https://www.economist.com/blogs/johnson/2013/05/vocabulary-size

an online site that tests your vocabulary size found that native English speakers who learned about and decided to test their vocabulary at that website had vocabularies ranging from 20,000 to 35,000 words.

Webster's New World College Dictionary--a small, cheap desk dictionary popular in the U.S.--contains over 80,000 words. The "Unabridged"--the Merriam Webster Third New International Dictionary, which is probably the most authoritative dictionary in the United States, has over 400,000 entries. The Oxford English Dictionary is said to have over 170,000 words. Because it's hard to know how to count "words" and "entries" (are "teach" and "tought" one word or two?) it's hard to be precise, but obviously most native English speakers know only a small fraction of the words in the dictionary.

To give another example,  if I open the Official Scrabble Player's Dictionary, 4th edition, at random, and look down the first column of the page, among the forty words I find fourteen words I don't know: hoverfly, howdie, howe, howf, howff, howk, howlet, hoy, hoya, hryvna, hryvina, hubbly, huck, huckle. And my computer is underlining twelve of them in red, so it doesn't know them, either.

The vocabulary test itself is at http://testyourvocab.com

Yes, I was too absolute, and "it depends on the dictionary" is a good answer.

In high school, every classroom had a good "college desk dictionary" of the 80,000-100,000-word variety, and every English classroom had "The Unabridged," i.e. the big Merriam-Webster... second edition when I was in high school. So that it is what I think of  when someone says "the dictionary."

But, even if we take at face value that a typical native English speaker, even a well-educated one, only knows a fraction of the the words in "the dictionary," it's hard to say what that means. It may just mean that because English has no central authority, and because of the example of the Oxford English Dictionary, the tradition is for dictionaries to scoop up as many words as they can possibly find, and that we simply may like to have our dictionaries include more obscure words in them than in other languages.

The heritage from two sources, Anglo-Saxon and Latin, does help to swell the count. That probably isn't unique to English, but I don't know what would be other examples offhand.

What can be said, and I am curious to hear what other posters think, that it is very rare for me to encounter a word I don't know--perhaps it happens once a month--except when I am playing Scrabble! Perhaps once a week I encounter a word I know, and know roughly what it means, but not a really precise definition.

For example, last year a member of the White House staff, asked when the President would keep an apparent promise he seemed to have make about releasing his tax returns, said He's not going to release his tax returns. We litigated this all through the election. People didn't care." To me, "litigate" means something that happens in a court of law, and I had to go to the dictionary to find out whether it could also just be a synonym for "argued" or "debated."

I am fascinated by Old English, too. It seems to greatly resemble German.

www.youtube.com/watch?v=jvgYLAKpU5g

Sepideh, in English we have the old-fashioned word "farewell" which is an alternative to the more modern "goodbye". I love that word. I think it sounds very nice.

It comes from the verb "to fare" and the adjective "well".

The original way to say it was to say "fare thee well" (singular) or "fare ye well" (plural).

That dates back to the time when English made a distinction between the singular and plural second person. "Ye" = plural "you" and "thee" = singular "you".

In some English dialects, people still retain those pronouns. For example, in Yorkshire (in the north of England) when speaking in dialect, people still say "thee". However, in standard modern English, most people don't use those archaic pronouns any more. I think that's a bit of a shame. They serve a very good purpose. They help us discern whether the speaker is addressing one individual or a whole group.