Japanese Particles Explained: は vs が, を, に, で and the Full List
You can know every word in a Japanese sentence and still not know who did what to whom. The words don't tell you. The particles do — the little hiragana stuck to the end of each word: は, が, を, に, で.
They are the first real wall in Japanese, and the one most learners hit twice: once on day one, and again a year later when は and が still feel like a coin toss. This is the whole system — what each particle does, the list you actually need, and a way to decide between は and が that isn't a coin toss.