Learn French: Ils boivent de l'eau pure dans le monastère historique. - They drink pure water in the historic monastery.

Yes, pur (or pure) is one of those adjectives that change meaning based on placement! When placed after the noun, as in de l'eau pure, it means physically pure, clean, or unmixed. If placed before the noun, it means 'mere' or 'absolute,' as in un pur hasard (a mere coincidence).

Why is it de l'eau instead of just eau or l'eau?

De l' is a partitive article, which is used for uncountable quantities. In English, we just say 'They drink pure water,' but in French, you must include the article to mean 'some' water. Because eau starts with a vowel, the feminine partitive article de la becomes de l'.

I know boire is irregular. Why is the 'they' form boivent and not something like buvent?

Boire (to drink) is a tricky irregular verb that uses two different stems in the present tense. For je, tu, il/elle, and ils/elles, it uses the stem boiv- (so, ils boivent). However, for nous and vous, it switches to the stem buv- (as in nous buvons).

Does the adjective historique also change meaning if placed before the noun?