Question

describe when using a dictionary is more appropriate than using a thesaurus

Answer 1

When you need the definition on a word, not other words for it.

Answer 2

A dictionary gives the meaning of a word. A thesaurus tells you the word in different ways but has the same meaning of the word you want. for example, if you look up sad in a thesaurus, it would probably say: SAD: UPSET, TEARFUL, DREADED, etc.