The most important thing is why you learn the language. If you want to write, grammar and style are very important. If you want to communicate, more important are vocabulary and pronunciation: you do not need grammar that much. This is at the very beginning of learning. After some time every single aspect of the language is equally important. It also depends on you native language and how you can manage to talk in English. There are some nations for which English is very difficult in pronunciation.
Note this is the view of a non-native English speaker.
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