Complete the following with appropriate articles: 

Sanskrit is one of _______ richest and most difficult languages in the world. It was used in _______ ancient times. 

The English language is distinguished by three articles, and they can be classified into definite and indefinite articles. While “the” is regarded as the definite article, “a” and “an” are considered indefinite articles. “The” is used in instances when we talk of a particular place, person, or thing and when we mention a singular noun to represent the whole class(Eg. The heart beats).  
“A” usually precedes a consonant(A ball, A pharmacy) or a vowel with a consonant sound(Eg. A yak, A university), and “an” is usually succeeded by a word that begins with a vowel(Eg. An astronaut) or a silent “h”(Eg. An hour). So, the correct way to frame the sentence is, “Sanskrit is a difficult language.”