One common pattern for this is the reason English has a comparatively simple grammar. English was learned in adulthood by a series of invaders (Vikings, Norman

One common pattern for this is the reason English has a comparatively simple grammar. English was learned in adulthood by a series of invaders (Vikings, Normans). Adults aren't very good at learning new languages so they tend to discard the harder parts. Indonesian (Bahasa) is easy to learn for similar reasons, while some of the indigenous tribal languages of Indonesia are among the most complex in the world.