Oh, you need to login? Hm… I didn’t expect that. I translated one paragraph somewhere on these forums… Never mind, I’ll just copy the translation and chuck it into a ‘hide details’ section here for you to read:
Translation/explanation of the ARC Academy article by someone else (not me)
5 categories of usage difference between は and が
Now, the question on the difference between は and が is so wrong, that it becomes a research topic for Japanese linguistics. One such scholar is 野田尚史 (Noda Hisashi). He published a book (1996) called 「は」と「が」. He categorized the difference between using は and が in 5 categories, which I feel is by far the most successful explanation on this topic. I am taking the example sentences and explanations from this page.
- New information vs old information
If the subject is new information, use が. If it is old information, use は.
鈴木（すずき）さんは校長（こうちょう）です。 鈴木さん is old information, already known to the listener, so は.
鈴木さんが校長です。 鈴木さん is new information, not yet known to the listener, so が. - Phenomenal sentence vs conclusive sentence
For a sentence that describes something that happens/has happened (phenomenal), use が. For a sentence that describes the subject’s property or opinion (conclusive), use は.
犬（いぬ）が寝（ね）そべっている。(phenomenal sentence) The speaker is describing what he has seen - that the dog is stretching itself out. So が is used.
それは私（わたし）の傘（かさ）です。(conclusive sentence) The speaker is talking about the property of subject それ = 私の傘. So は is used. - Whether the subject applies to the whole sentence or a clause only
が is used if the subject applies to a clause only. は is used if the subject applies to the whole sentence.
父（ちち）が晩酌（ばんしゃく）をするとき、つきあう。 The subject of つきあう is I. The subject 父 only applies up to 晩酌をする. So が is used.
父は晩酌をするとき、冷（ひ）や奴（やっこ）を食（た）べる。 The subject of both 晩酌をする and 冷や奴を食べる is 父. So は is used. - Contrastive vs exclusive
は is used to contrast the subject with another noun. が is used to show exclusiveness of the subject to the description.
犬（いぬ）は好（す）きだが、猫（ねこ）は嫌（きら）いだ。 - 犬 contrasts with 猫. So は is used. Note that often half of the sentence is omitted, so sometimes we need context to understand the は is used for contrast.
私（わたし）が責任者（せきにんしゃ）だ。 - It says that exclusively I am the PIC, not anyone else. So が is used. - Specificational sentence vs predicational sentence
For specificational sentence “AはB,” it is showing A is equivalent B. It can be reversed as “BがA”. For predicational sentence “AはB,” A is only a subset or property of B, so it cannot be reversed as “BがA”. は is used in predicational sentences, while both は and が are used in specificational sentences.
地球（ちきゅう）は、太陽系（たいようけい）の惑星（わくせい）だ。 The subject 地球 is only a subset of the predicate 太陽系の惑星 (predicational). So は is used.
あの人（ひと）の趣味（しゅみ）は、勉強（べんきょう）だ。 あの人の趣味 = 勉強 (specificational). Both は and が are used.
The original post is much longer, but I think those are the only relevant bits.
Not really, even though that doesn’t sound wrong. I’m just worried it might be too vague. Here’s my summary from quite a long time ago:
が vs は
- New vs old info (for answering questions like what, who, where…)
- Fact/phenomenon vs judgement/opinion
- Linked to start of sentence vs related to whole sentence
- Exclusive characteristics vs comparison
- Equivalence (pointing) statements (AはB can be turned into BがA and still make sense) vs descriptive statements (AはB makes sense, but BがA doesn’t)
For that last category, it’s more accurately ‘identifying statements’ (like A=B) vs ‘assertion/proposition’ (A is B, with B being a property of A).
Original Japanese terms:
新情報と旧情報
現象文と判断文
文末まで係るのか、節の中だけにしか係らないのか
排他と対比
指定文と措定文
EDIT: @hoaqin Just noticed a small but fairly significant error in the translation/explanation under the 5th category. I’ve made changes and put them in bold. Sorry for the quality. I would translate it myself, but I felt it would take too much time.