In comparison to the other languages, I think English is much more simpler. For example, compared to French, English nouns have no gender, adjectives have only one form and verbs have extremely simple conjugations. I'm wondering if there is a historical reason behind it. Has there been any scholarly research about the relative ease of learning the world's major languages, and if so, how does English compare to the others?
It started out as a pidgin, a simplified version of Anglo-Saxon that Norse invaders could understand. Anglo-saxon was(is) highly inflected, "the king gives horses to his men" can be written in many different ways. "king giveth horses", "horses are giventh by king" "by king horses givening" (Not really Anglo-saxon but you get the idea).
Now imagine you are a Norseman who knows the words,
king (because your language is also Germanic) but you can't pick up the endings. You have no idea who gave what to who. So if you are trying to buy cows from a farmer you need to simplify it to "me give you money, you give me cows".
Then after the Norman conquest all the people who cared about grammatical rules and correct language (monks, poets, grade-school teachers) spoke Latin or French, leaving English to be spoken by the normal people. Who just dropped complicated inflections when they couldn't remember them, or the most common endings were applied to all cases. Sometimes irregular endings have been kept as modern plurals simply because that was the most common usage.
Today, although the spelling is a bit random, English is easy to understand because it is so highly redundant. There are many ways to say the same thing, so even if you might not get a phrase exactly correct (to EL&U standards) you will be likely to be understood. In languages with a much more grammatical structure, where who is doing what to who (or whom?) is based solely on a suffix that is easy to get wrong - it is much more difficult to be understood.