More importantly, grammar is very important for English learners. If you study grammar solidly this summer vacation, it will be of great help and function to students who study English in the future. If you learn to write earlier, you will take a big step ahead of others. As for the teaching materials, you can choose the first or second book of New Concept English for Teenagers. Cambridge English may be a little easier for them, but it's not bad. There are exercises that are sold in bookstores. Thank you. Please take it. Phonetic symbols are also very important, which is related to the oral test in the future.