Lesson 3: Particles and grammatical case
Japanese has "particles". Particles are similar to prepositions. They mark an entire phrase as having some logical relationship to something else. Japanese particles come after the phrase they're modifying instead of before it.
English uses word order when you need to know what job each noun has in a sentence: subject, verb, object. This can get flipped around in other patterns and in poetry, but word order is english's main thing here.
These jobs are called grammatical "cases", and each job is a particular "case".
Japanese uses particles to indicate these jobs. There's a default word order, and you can drop particles, but particles are the norm.
Remember: these translations are only for demonstration, and the japanese sentences are not necessarily natural-sounding. The sentences and translations are only here for illustration, not instruction.
ジムがネコを食べた
じむが ねこを たべた
Jim ate cat.
Here, が marks "Jim" as the subject. を marks ねこ as the direct object. The subject of a verb like "eat" is the person doing the eating.
Japanese throws a wrench into the picture by having something we call a "topic marker", は, which literally just says "this is what I'm talking about". Most grammar resources compare は to が. They do this because sometimes it's unnatural to use が, and you have to use は instead, or leave the subject unstated. But は is more general than being an alternative to が.
ネコはイヌが食べた The cat, a dog ate it.
イヌはネコを食べた The dog ate a cat.
One way to differentiate は and が here is thinking of が as a focus marker. This "focus" is in addition to が's behavior as a subject marker, not instead of it.
ジムがネコを食べた Jim is the one that ate the cat.
English uses articles and swip-swappy sentence patterns for this kind of focus.
ネコは魚食べる
Cats eat fish (cats in general)
The cat eats fish (the cat we were talking about)
ネコが魚食べる
A cat eats fish (about a particular arbitrary cat)
The cat eats fish (about a relevant cat)
Cats are what eat fish (cats in particular)
Japanese is a "subject-object-verb" (SOV) language. That is, normal statements have subject, object, and verb in that order. English is SVO. Both languages allow moving the parts around, but doing so changes where the emphasis goes.
If you need more information on は, view "Unit 8: Particle wa" of "Visualizing Japanese Grammar".