Five small words carry most of Japanese grammar's heavy lifting.
Here's what each one actually does.
Japanese doesn't rely on word order the way English does. Instead, small words called particles (助詞, joshi) attach after nouns to mark their grammatical role — what's the topic, what's the subject, what's being acted on, where something happens. Get the particle right, and the sentence structure mostly takes care of itself.
This is the pair that trips up almost every learner, because English doesn't really distinguish them. は marks the topic — what the sentence is about, setting the stage. が marks the grammatical subject, and tends to introduce new information or directly answer a question.
を marks the thing directly receiving a verb's action — what's being eaten, read, watched, bought.
に does more work than any other particle on this list. Its core uses:
で marks the location where an action takes place, or the means/method used to do something — different from に, which marks a destination or the location where something simply exists.
Particles are best learned through repeated exposure to real sentences rather than memorized as abstract rules. Reading example sentences out loud, and noticing which particle shows up in which context, does more long-term work than drilling grammar charts alone.