You can tell whether zij means she or they based on what verb is used. For example:
"Zij drinkt dure wijn."
"Zij drinken dure wijn."
The first one would be she because the verb "drinkt" is used, whereas the second one would be they because the verb "drinken" is used. It just depends upon the conjugation.
"duur" is used when the adjective proceeds the noun or, in the case that it precedes the noun, if there is no definite article with a "het" word. In every other case "dure" is used. Some examples:
- Wijn is duur.
- Bier is duur.
In these cases the adjective proceeds the noun, so "duur" is used. Whether the noun is a "de" or "het" word does not matter in this case.
- Zij drinkt dure wijn.
- Hij drinkt duur bier.
In both of these examples the adjective precedes the noun, and there is no definite article. When this happens, "dure" is used with "de" words and "duur" is used with "het" words.
- Zij drinkt de dure wijn.
- Hij drinkt het dure bier.
In these examples the adjective precedes the noun, but there is a definite article. "dure" is used regardless of whether the noun is a "de" or "het" word.
These rules apply to most adjectives. You can find an explanation for this in the first adjectives skill. :)