English is difficult to learn because of how much it combines and then changes things from other languages, then makes exceptions. Most vocabulary and grammar can be learned similar to other languages, but there are some things that can only really be learned through exposure and immersion.
"There is a beautiful, large, old, brown dog" is a sentence that requires that specific order. While technically acceptable, it sounds incorrect to say "A brown, old, large, beautiful dog".
There's also a lot of exceptions to rules. For example, the words "read" and "read" are pronounced differently based on tense, but "our" and "are" can be pronounced the same but only in certain accents/dialects. To make things even more complicated, in regions where "our" and "are" are not pronounced the same, in those areas "our" and "hour" usually are.
Most language learning apps that are game based, such as Duolingo, can't really account for these weird rules. I've heard good things about Babel and Rosetta Stone, but there's not much of a substitute from being in an environment where your only option is to speak the language.
If you can, get your basics, then practice with others. Additionally, the general rule is that US media is designed for a language comprehension level of 6th grade (11-12 years old). That's usually the expected vocabulary goal of a fluent and native English speaker. Anything beyond that is usually just for academics or historical analysis such as understanding Shakespeare.