Again:When to use nouns to modify nouns and when to use adjectives to modify nouns? No one helps me?
When to use nouns to modify nouns and when to use adjectives to modify nouns? For example: "experimental gadgets" or "experiment gadgets"? "architecture design" or "architectural design"? "science historian" or "scientific historian"? "science Americans" or "scientific Americans"? I have read "science historian" and "scientific Americans" in the passages, but why?
In these situations, in Chinese language, we all use nouns(by the way, in these cases of Chinese, nouns and adjectives are in the same form), so I am confused.

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I think there is no rule. Sometimes nouns modify nouns, sometimes adjectives modify nouns. Only an English dictionary, English grammar and English texts can help you. One more advice - don't compare English with Chenese. When people compare their mother tongue with another foreign language they usually apply rules of their mother tongue to the foreign language they are learning.

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I would say that the adjective should be the modifier in all your examples. If you can change the noun into an adjective, then do so. Examples of nouns modifying nouns include "door bell", "soup spoon", "mountain bike" etc. These words can't be changed into adjectives, whereas in "scientific Americans", "architectural design" etc, you can.

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Thnak you Harry.

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Here is my opinion.

I think adjectives are what modifies nouns. Sometimes you might have a compound noun made up of two nouns, like "science history". A person who studies science history (compound noun) is a science historian. I'm not too sure whether "experiment gadgets" is accepted widely in formal writing, but it sounds okay at least in conversation. In either case that would also be a compound noun.