How did the english language evolve to have an easier grammar?

The short answer is that we simply don't know.

Presumably you're talking about the change from Old English (the Germanic language of pre-1066) to Middle English (the language of Chaucer)? The problem is that the process was completely undocumented.

There's a very nice theory that English developed initially as a pidgin - a kind of cut-down hybrid language - enabling communication between the locals and ruling French. But it doesn't really fit; firstly you'd expect some documenter to have at least mentioned it, and secondly it doesn't match the timeline. From late Old English manuscripts, it's known that some of the processes of simplification were already under way in the a couple of centuries or so before the Normans invaded.

It's nothing to do with centralised education. Both before and after the Norman conquest, the great majority of the population was illiterate, and you shouldn't assume that the language of the peasant classes would have been unsophisticated and/or liable to rapid drift or degradation. The languages of many sub-technical cultures without writing systems or organised education are far more complex than English, and very stable in that complexity.

In Europe, Vulgar Latin, spoken by ordinary uneducated people, took around 1000 years to evolve into the early forms of Spanish, Italian, and so on, keeping most of the inflections, gender, etc, and these languages still are recognisably based on Latin. English radically 'rebooted' over 300, into a new language of near-zero mutual intelligibility with Old English. Something very different was going on.

It is hard to figure out everything all together... true... The main thing is that it was not a radical change, but a very gradual one, back then. Centuries going by is how we have to reckon this. The next is that although there were governments and authorities then, they had no concern at all for issues about the language, not really. (When the Normans 'came' to England, after 1066, they spoke an entirely different language than Old English, and didn't give a royal f* about what the people they didn't associate with, in lower classes, were speaking, or the shape of *that* language...) Next after that is that -- for centuries -- there really were no 'schools' in the modern sense: an extremely small number of people in the whole population got any formal education at all, or learned to write as well as speak. ... We'll jump back a few centuries for a nice explanation of one thing -- the exact video program I can't lay my hands on right now.... The Scandinavians (the Vikings, if you will) took over a whole big chunk of England in the middle east and middle north of what was to become England. This was called the 'Danelaw' area. The nice video part was that, since Old Scandinavian and Old English had a lot of root words in common, they were all close to understanding each other -- in a lot of ways. So both sides just dropped a whole lot of the declination endings, etc., that were different. Makes sense. But these SORTS of things, were LONG and SLOW processes, taking hundreds of years. All that is oversimplifying the evolution, but has the basic evolution of the whole thing in place...

In Spanish there are at least 2 ways to pronounce a vowel including A. English,Dutch and Danish have simplified their grammar during the last few centuries.Spanish,German and French have not.Of course English spelling often is not phonetic according to English spelling rules.Examples are;enough,though,through.This is one of the major problems with English.

England became a colony of France and for 300 years English became merely the despised "patois" of the serfs in the fields and the household servants. It survived in the form given to it by the uneducated masses because many of the new French conquerors were young unmarried knights on the make who shacked up with local concubines who used this "barbarous" jargon in talking to their charges in the nursery: exactly the way Guarani has come to triumph over the official Spanish in remote Paraguay. Whether the clumsy language that we now use has an "easier" grammar is open to debate. It manages to combine a lack of the semantic clarity of French, of the melodiousness of Welsh or Italian, of the flexibility of academic German, of the in your face directness of European Spanish and of the laconic brevity of Latin, with an orthographic complexity beyond that of any other European language (with the possible exceptions of Danish and Irish Gaelic).

Order out of Chaos ...