Translation:This book is short.
Would someone be able to clarify when exactly the は and が particles are used? I've been studying this language since grade 8 so I'm definitely not new to this and so I know generally when something 'is' something else は is used, however, I have noticed here sometimes the particle が is used instead and I'm not entirely sure why.
(Disclaimer: I most likely have less experience with Japanese specifically if you have studied it from grade 8. Much of what I’m saying is originally extrapolated from other topic-prominent languages, most importantly Mandarin and Korean, but those extrapolations fit my experience with Japanese remarkably well.)
Very briefly speaking, は marks the topic, the thing you are talking about, while が marks the grammatical subject of a particular sentence. If the topic happens to also be the subject or object, は replaces the subject marker が or object marker を.
Now what is the difference between a topic and a subject?
Basically, the topic is something which the speaker wants to put in the focus of discourse, maybe to put contrast on it, or maybe in order to be able to imply it down the line without having to repeat pronouns over and over again like we have to do in English. So it stands somewhat unconnected to the sentence and is more of a signal for “I’m going to talk about x now”. Of course it can happen to also have a grammatical role in the sentence in which it is introduced (I’ll mark topics bold and put implied things in angled brackets in the translation):
- 僕はこの本を読みました。 ‘I read this book.’ (topic = subject)
- 野菜は食べません。 ‘[I] don’t eat vegetables.’ (topic = object)
- アメリカには行きたくない。 ‘[I] don’t want to go to America.’ (topic = location)
But the topic can also something which isn’t a grammatical sentence constituent but just something that is commented on. The only way you can emulate this in English is by using a phrase like “as for…, concerning…” (although very often the translation is more natural if you rephrase the sentence in English):
- 彼は僕と背が同じくらいだ。 ‘As for him, the height is the same as me. (or in better English: He is as tall as me).’
- 僕は子供がいません。 ‘As for me, there are no children. (=> I have no children).
The topic is something that is maybe better understood as something which belongs to the discourse, not the sentence. You say “xは” and then you can go on talking about x. x will be implied where there is information missing (e.g. when people hear 背 ‘height’, it is implied that the speaker is talking about the height of the topic). This implication carries over to following sentences. I once read an explanation using a story of an alien which something went like this:
“I saw an alien last night. It was really big but also very fluffy so I wasn’t scared of it at all. I wanted to go and hug it but it was so big I couldn’t wrap my arms around it.”
As you can see, the whole passage is talking about the same topic. It also exemplifies another feature of topics: They are generally “old news”, i.e. something that is already known to the listener. Often you see an introductory sentence (“I saw an alien last night.”) before the thing is made the topic. So in Japanese the story would begin like:
Last night I saw an alien! Alienは…
(To be sure, the topic doesn’t have to be introduced like that; it could also be something that is known to the listener anyways: A pronoun like “I”, a general category like “dogs”, a name the listener can be expected to know…)
So in order to fully understand topics and how they are used you have to look at more than just one sentence in isolation because topics are more part of the greater discourse. Unfortunately this makes them difficult to teach using Duolingo…