Hindi is a most commonly spoken and written language of India, Mauritius and to some extent in Nepal. The current form of the language evolved during late 1850s when most notably Bhartendu Harishchand and later on Hazari Prasad Dwivedi, Devaki Nandan Khatri, Premchand etc. contributed heavily to its literature.
Its roots are found around 750 AD, when its predecessors Prakrit, Pali and Brij Bhasha started morphing towards the current form. It was also the time, when Mongols and Middle Eastern raiders started focusing towards India. They were following on the footsteps of Huns who have been plundering and raiding India since 2nd century AD. There was one more fact, which significantly contributed to the early foundations of Hindi was, really the decline of Buddhism in India. After enjoying almost thousand years of its glory Buddhism was fizzling away. There were two reasons for this to happen, first reason was that Buddhism had met a spiritual impasse in its progress and the second reason was that Adi Shankarachrya and some of his predecessors had successfully rekindled the spiritual life based on Vedic teachings.
In order to understand and follow Vedas, the scholars and spiritually oriented people needed to have a firm grip on the Sanskrit language, but at the same time general public was repeatedly being raided by the likes of Mohammed Gauri, Mahmood Ghaznawi, Nadir Shah, Taimur Lung (750 AD – 1400 AD) and there was constant mixing of Arabic and Farsi speakers amongst them. Around 1450 onwards Arabic/Mogul raiders started realizing that instead of occasionally visiting India, it was better to settle in India and enjoy the riches year around. These settlers spoke Arabic and Farsi. It was not uncommon to have marriages between these two parties and therefore, even if the mother and the father spoke different languages, their offspring started making a mix of these languages.
What we know now as “Hindi” is basically a language whose grammatical construct follows the rules of original Sanskrit grammar but has generous sprinkling of Arabic and Farsi words in it.