1. Its really necessary to learn grammar because you have to know how to use the words, which tense to use and to show the level of your English.
2. I think in isolation because they are completly different things. Vocabulary is for learning the words and phrases. Grammar is for how to use those words and phrases.
3. It's better to practice using new words because you commit to memory better when you pronounce those words and repeat them.
4. They are not easy to learn because they are quiet long. But anyway you hve to learn and use them because Englishmen appreciate knowledge of some funny idioms.
5. Because there are a lot of them and they are changing according to the time you live. So, you always have to get new ones for learning and using them in your speech.
6. It's important because they listen to you and they should understand you. For example, you say positive sentence with the innotation for negative or question sentences.
7. It's quiet good for your logic. You have to think in English.
8. You have to try to catch the most imporntant words and the words you understand. Then, you just need to use your logic.
9. I think it's grammar because you have to learn it by heart, you can't practice it with people.
10. Social English is the language of everyday communication. For example, when you are talking to your friends at school.