The best way to learn Japanese is through a variety of methods. You need to work on your grammar, listening, speaking, reading and writing. Its best to think of these as separate categories and focus on each specific category as these require different methods and techniques.
Start by learning the basics through grammar. These books really helped me:
• Shin Nihongo no Kiso I & II
• An Introduction to Modern Japanese by Osamu Mizutani & Nobuko Mizutani
• Japanese Demystified by Eriko Sato a self-teaching guide
These books cover both polite Japanese, which is extremely important to learn in order to speak Japanese effectively and causal Japanese, which is great for everyday conversations.
I also recommend trying to learn a certain amount of vocab or kanji each day as this helps you build up your vocab and kanji ability. I tried to learn 5 kanji per day or about 20 a week and did the same with vocab.
Watching Japanese anime or any Japanese TV program is a good idea for improving your listening skills and also your vocabulary. Keep a notebook and jot down any new words you hear. Then look them up and find out the meaning.
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