「は」と「が」の違い/「は」と「が」のちがい/”Ha(wa)” to “ga” no chigai
Distinguishing the difference between the Japanese particles "ha" and "ga" is said to be very difficult. You are probably using these two unconsciously without differentiating them. There are only a few people who consciously use は and が in their writing. As a grammatical explanation, が is a case marking particle and "ha" is categorised as both conjunctive particle and adverbial particle, and they are completely different things. However, in actual sentences, there are many situations where は and が can be used interchangeably in a sentence, and there seems to be no difference.
So, how should we use the particles は and が differently? Japanese language researcher categorised the method of differentiation of は and が into 5 types.
The first method is to differentiate whether the information is new or old. In the following two sentences, は and が are used, but if you look closely, the nuances are different.
富士山はきれいだ/ふじさんはきれいだ/Fujisan wa kireida/Mt.Fuji is beautiful
富士山がきれいだ/ふじさんがきれいだ/Fujisan ga kireida/Mt.Fuji is beautiful
The sentence with は is used when the speaker wants to say, Mount Fuji is beautiful in general terms. It is used in the situation where Mt.Fuji is not there and giving the information that everyone knows. On the other hand, the sentence with が is conveying new information to the listener in a situation where the mountain is right in front of him. To summarise, in conversation or in context, は is used when the genetive noun of the sentence is already known and が is used when it is unknown.
The second method is to differentiate depending on whether the sentence is phenomenal or judgmental. The phenomenal sentence is a sentence that expresses a phenomenon as it is, without adding the speaker's subjective judgment. On the other hand, the judgemental sentence is a sentence in which the speaker expresses a subjective judgment about a phenomenon. If the sentence is phenomenal, the postpositional article should be が and if it’s judgemental, は is used.
(Looking at the cat) 猫が寝ている/ねこがねている/Neko ga neteiru/The cat is sleeping
This is a phenomenal sentence because it’s describing what the speaker saw without adding subjective opinion or judgement.
(Pointing at your stuff that was almost taken by someone else) それは私のものです/それはわたしのものです/Sorewa watashi no mono desu/That’s mine
This is a judgemental sentence because it’s claiming that stuff belongs to speaker after speaker saw someone almost took it.
The third method is to differentiate depending on to what extent subjective case (the subject of the sentence) is the focus of the sentence, whether it is focused all the way to the end or only in the clause. If the subject of the sentence is focused all the way to the end, は is used and if only in the clause, が is used.
The fourth one is to differentiate by according to whether subjective case shows contrast or exclusion. は is used when it has the meaning of contrast, as in “compared to this (another noun), it (subject) is ___”. が is used when the meaning is exclusive, as in “only this (subject) is ___”
猫は苦手だが、犬は好きだ/ねこはにがてだが、いぬはすきだ/Neko wa nigate daga, inu wa sukida/I don’t really like cats but I like dogs
This is comparing sentence so は is used for both nouns.
私が責任者だ/わたしがせきにんしゃだ/watashi ga sekininsha da/I’m in charge here
This is an exclusive sentence because it expresses that I alone am in charge, not the others here.
And lastly, the fifth method is to differentiate by the types of the sentence. There are two types of sentences called 指定文/していぶん/Shiteibun and 措定文/そていぶん/Soteibun. These two types differ by the placement of predicates and subjects. Predicates are the part of a sentence or clause containing a verb and stating something about the subject, and the subject is a part of a sentence that contains the person or thing performing the action (or verb) in a sentence.
蜘蛛は害虫です/くもはがいちゅうです/Kumo wa gaichu desu/Spiders are vermin
害虫は蜘蛛です/がいちゅうはくもです/Gaichu wa kumo desu/Vermin are spiders
This is an example of 措定文(soteibun). The first sentence is the correct sentence and the second one is reversed one but it does not make sense. In such cases, は is used.
佐藤さんはあの人だ/さとうさんはあのひとだ/Sato san wa anohito da/Mr.Sato is that person
あの人が佐藤さんだ/あのひとがさとうさんだ/Anohito ga sato san da/That person is Mr.Sato
This is an example of 指定文(shiteibun). As you can see, both sentences make sense both in Japanese and English. In this case, both は and が is used.
This article explained the difference between は and が. It’s a difficult concept and often native Japanese speakers confuse. It's such a big topic that many research papers have been written on it, and it's confusing to find different information on different sites, but I hope this article is helpful in your writing.