for instance: economy project industry project; but economic problem industrial problem And construction problem, construction project; why can't i write economy problem?what is different between project and problem?
You actually can use economy problem.
As a matter of fact, according to Google Books NGram Viewer, economy problem is slightly more common in print than economical problem:
Generally speaking, adjectives are used more often in front of nouns. (That is, after all, their syntactic function.)
However, nouns can also be used in an adjectival fashion. When used in this way, they are called attributive nouns.
Mignon Fogarty says this in "Noun or Adjective?":
. . . although we have adjectives in English, we can also use nouns as adjectives. When we do so, they’re called attributive nouns.
Not all nouns have related adjectives. “Cotton” and “fleece,” for example, are your only choices for describing a cotton shirt and fleece jacket. But when there is a related adjective you get to choose. For example, since “wool,” and “silk” have the adjective forms “woolen” and “silken,” you get to choose between the attributive noun and adjective. You can wear a silken scarf with your woolen sweater, or you can wear a silk scarf with your wool sweater. Both ways of saying it are correct. You can also mix and match, saying you wore a silk scarf with your woolen sweater, but I think it often sounds better to stick with the same form within one sentence.
As for what may be different between one phrase and another, and why we choose one form over another, general patterns of grammar will apply—but English is full of examples of idiomatic usage that don't fit into a logical structure. There are many things that we say simply because we do.
Noun and adjective are two different things. There are set of words that can be interchanged to be noun or adjective . Adjective, are words basically used to describe or modify the noun. Examples of which are a graceful dancer, eight-year-old kid, grumpy old man. So noun is only a word but adjective is its modifier in the sense of its description.