The path to being able to use the English language well is long for most people. Native English speakers have spent their entire life learning English. Most native speakers learn so gradually that they aren't really aware that they are continuously learning!
Non-native speakers normally have a shorter and harder path. Learning English As a Foreign/Second Language (EFL/ESL) is something that is hard for most people.
The English language has evolved a lot in the last thousand years. We have adopted structures and words from other languages. There are rules (grammar/form) for how we use English, but there are more exceptions than rules. There are more than a quarter million words in the Oxford dictionary, some of which are obsolete. Knowing the definitions (meaning) of all of those words is not possible. Many of those words were adopted and/or adapted from other language. Pronouncing English words (pronunciation) can be challenging. There are sounds in English that don't exist in other languages. As well, because we have words originating in other language, we use the source language's rules to say the word (think muffin vs buffet, we say the n but not the t). Further, we use different communication styles in different contexts. How you talk to your friends on the basketball court is very different from how you would speak to the Queen of England (say "hey, lady, how y'a doin'" is probably not going to go over very well. Learning what language to use in what context (appropriacy) takes a lot of practice and awareness.