English is constructed out of every other language from Europe.
We had Latin from the Romans, Celtic, Gaelic, Old French, Middle French, Anglo-Saxony, even German and other assorted Nordic languages creeping in.
Basically it means that while many languages evolved from one source (for example, the Romantic languages, a series of languages which all emerged out of Latin; Japanese, Chinese) 'English' was never exactly a language. In its earliest stages it was akin to pidgin (a language constructed from words you understand that the locals use and words of your own language) in order for the various conquerors to communicate with the natives.
But, this is also why it quickly became one of the dominant languages once England stopped getting invaded and started doing the invading, back in the days of Old English and Middle English it would be quite possible for a Frenchman and an Englishman to talk to each other in their own language and understand large amounts of what was said (similar to how Portugese, Italian and Spanish speakers can usually understand each other with minimal effort) Its only when proto-Modern English starts cropping up that we started to take massive steps away from other languages.
So that's basically it, when you learn Modern English you aren't learning any one language, you're learning a whole cavalcade of languages with different syntactic incongruities (English has no male and female nouns for example).