Firstly, English learners are required to memorize a lot of vocabulary To be able to express their ideas accurately.Learning  so many words enable them to have a variety of alternatives to use while speech.Secondly, mastering the grammatical rules enable them to deliver the meanings in a correct way that helps recepients understand the desired meaning correctly.finally, learners have to learn how to pronounce the vocabulary well , otherwise the recepients will get the wrong meaning.For example, learners should differentiate between the letter (b)and (p)in pronunciation.The letter  p should be pronounced with a push of air.Imagine what native speakers would understand if you mispronounce the letter b in the following sentence

Dogs bark a lot during nights.