Many of the rules of grammar we observe today began in 18th century when several influential books of grammar were written.
In particular Samuel Jonson developing his dictionary of English in 1755 which brought in standardisation to not just spellings but also definitions and meanings.
It also confirmed the Midlands accents (Oxford and Cambridge) as the PREFERRED way of writing and spelling
The 18th century also had standardisation in the growth of education and literacy - many accepted regional expressions and phrases were replaced by standardised ones - particularly in writing.
The invention of printing with Caxton in 1476 created a requirement for standardisation as printers were competing with each other.
Caxton himself chose to print texts in the East Midland dialect - London, Oxford, Cambridge as these were seen as the most prestigious and "correct" form of English
Printing also impacted in that spelling and punctuation became more standardised and in the 17th century a modern punctuation system began to occur.