If you're asking about the difference between "Wij lezen een oud boek." and "Wij lezen één oud boek.":
"Wij lezen een oud boek.": Here "een" is the indefinite article; it's pronounce with a schwa (vowel with no specific sound). We're reading an old book.
"Wij lezen één oud boek.": Here "één" is the number (one); it's pronounced /eːn/, similar to English "vain" without the 'v'. We are reading one old book. The accents are written when it's not clear from the structure of the sentence that it can't be the article "een". (Of course, accents are also written when the word is stressed, usually that too happens for the number, not for the article.)
If that's not your question, could you elaborate a bit more?