Particles and grammatical case
Let’s look at more complex “A does B” type of statements. We will introduce verbs and conjugations in a later lesson, so take the example sentences used here just as a general guideline, don’t try to break them down.
To understand “A does B” sentences, we first need to introduce “particles” and the concept of loose word ordering.
Japanese uses "particles" (助詞), something similar to prepositions, to define the relationship words have with other parts of the sentence. Japanese particles come after the phrase or word they're modifying instead of before it. We say particles “attach” to the word to their left. We already saw something similar to particles in the previous lesson: the だ copula.
In English, we use word order to understand what role each word has in a sentence: subject, verb, object.
These roles are called "cases", and each role is a specific "case". Japanese uses particles to indicate these roles, that’s why they are often called “case-marking particles”. We say a particle “marks” a role when we want to say that the word it is attached to assumes said role in the sentence.
There's a default word order, and sometimes particles can even be omitted if the meaning can be inferred from context, but in general particles are the norm. Some words can also exist “freely” in a sentence without particles, we’ll take a look at them in a later lesson.
Remember: these translations are only for demonstration. The sentences and translations are only here for illustration, not instruction.
ジムがネコを食べる Jim eats cats.
Here, が marks "Jim" as the subject. を marks ねこ as the direct object. Then the verb comes at the end of the sentence. The subject of a verb like "eat" is the thing doing the eating. The object is the thing that is being eaten.
As introduced in Lesson 0, it is more common to first introduce a “topic” rather than explicitly mark a subject or object. To do this, Japanese uses the “topic marker” particle 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 do not be tricked, は is more general than being an alternative to が. Sometimes it replaces が in the sentence as it introduces the subject as being the topic, but other times it can mark the object as a topic instead. In those cases, the thing marked by を is what gets replaced. As seen in the following examples.
ネコはジムが食べる The cat, Jim eats it. ジムはネコを食べる. Jim eats a cat.
Since the default expectation is to first introduce a topic, and then describe it with the rest of the sentence, when we intentionally choose to explicitly make use of non-topic particles (like が) where a topic would be expected, it ends up putting an additional focus on that part of the sentence. This "focus" is in addition to が's behavior as a subject marker, not instead of it. It is common for が to have this nuance, but not all が are focus markers, sometimes they are just neutral subjects and the topic is something else.
Compare the following:
ジムがネコを食べる Jim is the one that eats cats. ジムはネコを食べる Jim eats cats.
English uses articles and funny sentence patterns for this kind of focus, but Japanese needs to rely on particles, because the order doesn’t matter grammatically.
Another difference is that the topic は is often assumed to be something that has already been introduced in a conversation (similar to “the” vs “a” in English), or something that is universally understood as a general statement.
For example, the following translations are all valid. Context will make it clear which is the most appropriate.
ネコは魚を食べる Cats eat fish (cats in general) The cat eats fish (the cat we were talking about) ネコが魚を食べる A cat eats fish (a particular arbitrary cat) The cat is the one that eats fish (a specific cat out of a group)
This lesson is only scratching the surface of the various roles は and が can assume. Do not try to memorize them as universal truths, just be aware of the general vibe and meaning these sentences can convey when you see them in context.