Indeed, my interpretation is also that the subject can be expressed by any of は, が, or the context. I am having trouble understanding the "implicit が" analysis because it seems to me that in many cases adding a が would make the sentence nonsensical (like in the example.) So it just feels like retrofitting が into a role that it really doesn't have. Then again, I'm not an expert, so I may very well be wrong about this.
That's why it's called a zero-が if there's no "が", because every valid Japanese sentence needs a subject. It may sound like nitpicking, but it's important to clarify that it is not correct to say that "the subject can be expressed by any of は、が, or the context". Thinking about it that way only makes it difficult to understand where the particles should be used, and indeed the difference between は and が. Either the subject is explicit and marked with ”が" and only ”が", or it's implicit from the context. If it's unclear what the context is, you can clarify the context by introducing a topic with ”は", but that does not in any way make は itself a subject marker.
You're entirely right, what I said there was incorrect (or sloppy at best.) Thank you for the correction.
> That's why it's called a zero-が if there's no "が", because every valid Japanese sentence needs a subject.
This is the part which I find unhelpful. You could say "every valid English sentence needs a subject and a verb", and you'd be having a hard time finding a sentence in e.g. the U.S. constitution that doesn't explicitly contain a subject and a verb.
But in Japanese, not every sentence needs to specify the subject. Every sentence still has a subject, because the definition of a sentence is "a subject and a predicate", but that applies to every human language.
The problem then is that the "zero-が" then feels like bolting a potentially ungrammatical (or at the very least unnatural) construct onto every sentence to explain the simple fact that yes, indeed, Japanese sentences do have a subject, but it's often unspecified. It also misses the important point that が, by clarifying the subject, also has the role of emphasizing it, which changes the meaning of the sentence.
To go on a bit of a tangent, Hungarian has a similar problem. It also frequently omits the subject, and specifying it also changes the meaning of the sentence:
* Lefekszem. =I'm going to sleep. (The subject is already specified by the conjugation.)
* Én lefekszem. =It's specifically me, who is going to sleep. (For example, you are asked if you're going to the after-party, but in contrast to others, you're tired and want to sleep.)
You could add an "implicit én" to the first sentence, to show that yes, it indeed does have a subject. Maybe it's helpful for people whose native language can't just omit the subject. But it subtly changes the meaning, so ultimately I don't think it's very good analysis, but rather "translating into your native language."
As I said, I'm not an expert, but I hope what I'm saying makes sense.
There are some sentences in English that use a zero pronoun as the noun, though it is generally in cases where it feels like the noun is already specified.
The main reason for imagining the zero-が is that it helps you understand the structure of Japanese. Do this, and you'll never get confused by "は" and "が" again. And understanding that が always marks a subject, and never anything else, also helps resolving some apparent special cases and exceptions (which aren't, in fact, exceptions) in Japanese. Specifically, が never marks the object, even if textbooks and even some Japanese teachers who have learned Western grammar think so sometimes.
I recall the explanation of an excellent little Japanese grammar book that made it click for me: 私は puts the emphasis on what you, as opposed to everyone else is doing, and the closest english equivalent is "As for me,".
If you translate it that way, 「私は牛肉を食べます」means, in English: "As for me, I will eat beef."
Notice now that both "me" and "I" are present in the translation. So adding が doesn't in fact change the sentence at all!
