how can we say that the Hindi language was not used in the ancient times

Dear Student,
Evolution of language is a constant process. We can say that Hindi was not used in the ancient times because there are no written proofs such as books, manuscripts, edicts or scriptures written in Hindi dating to the ancient times. 
However, the ancestors of Hindi i.e. Khadiboli,  Devangari Script based languages such as Sanskrit were prevalent. 
Regards