You posted in the wrong place, better if you select Italian language, so more Italians can see your question. 对你的问题, 你懂得错. Allora gli compriamo una bottiglia di cognac "Gli" means "a lui", it's not an article, it's a complement pronoun (代词). Another way to write it is: "allora compreremo a lui una bottiglia di cognac". 但是你的句比较自然. 明白了吗?