English is a hard language to learn, especially as an adult. So many weird rules that everyone takes for granted.
English has simple conjugations. That makes it much easier to learn to speak intelligibly even for adults. Old English dropped many of the complicated word endings because it was used by Anglo-Saxons with Vikings and Normans.
Understanding of grammar and rules of a language is like having good bricks, but building a stylish house is more than just using good bricks. It's a higher level cognitive exercise. Schools attempt to teach this, but i think they fail quite miserably (look at how essays had to be written by students, and how poorly most are written).