prompter: In Japanese grammar, what is the difference between wa (は) and ga (が) particles?

assistant: The Japanese particle 'wa' (は) is employed to indicate the topic of a phrase, while 'ga' (が) marks the protagonist of a phrase.
The particle 'wa' (は) is utilized to present a theme that is already known or established, and to show that the orator is varying the concentration of the dialogue. It is likewise employed to point out the usual theme of a piece of writing. For instance, in the phrase "Kore wa watashi no hon desu" (これは私の本です), 'wa' signals that the topic of the phrase is "this".
The particle 'ga' (が) is utilized to introduce a new topic or subject, and to emphasize the protagonist of a clause. It is also utilized to signify that the orator is making a declaration about the subject. For example, in the phrase "Taro ga hon o kaimashita" (太郎が本を買いました), 'ga' signals that the subject of the phrase is "Taro" and that the phrase is making a statement about him.
Further, 'wa' can be employed to signify a contrast between two topics, while 'ga' is utilized to signify a comparison between two topics. For example, in the phrase "Taro wa watashi yori atsui desu ga, watashi wa Taro yori tsumetai desu" (太郎は私より暑いですが、私は太郎より冷たいです), 'wa' indicates a contrast between "Taro" and "I", while 'ga' indicates a comparison between the two.
All told, 'wa' (は) is employed to introduce a theme and to signify a contrast between two topics, while 'ga' (が) is utilized to introduce a new protagonist and to indicate a comparison between two topics. By utilizing these two particles correctly, you can readily make your point lucid in Japanese.

assistant: Japanese particles は (ha/wa) and が (ga) are often picked out as the most confusing area of Japanese study, and it continues to be confusing right the way through from beginner to advanced level. 
From a grammatical point of view, が goes after a word or phrase to pick it out as the subject of a sentence, or part of a sentence (a "clause"). That is, the person or thing before が is doing or being something. It's the protagonist. Grammatically, it's a relatively defined and straightforward role. An example might be:
子供が 遊んでいる。(A child is playing.)
子供 (child) comes before が here, showing us that 子供 is the subject. In other words, 子供 is the person doing the action of "playing." 子供 is the "perpetrator," and we know this because が is helpfully making this clear.
は, on the other hand, isn't picking out a specific part of a sentence and telling us its exact grammatical role within that sentence. Rather than showing us our subject, or object, or some other grammatical building block, は is doing something quite magical that most languages can't do explicitly in this way: it's telling us the topic of our sentence, or part of a sentence, or maybe a whole paragraph or more. Here's an example:
東京は 賑やかな 所です。(Tokyo's a lively place. / As for Tokyo, it's a lovely place.)
Both wa (は) and ga (が) show us where emphasis lies, and in doing so they make a distinction between information that is already in our consciousness and information that the speaker or writer wants to introduce — or re-introduce — into our consciousness.

prompter: I still don't quite get it - you're making it seem like both particles are interchangeable, but at the same time not. So, just to make things clearer on a concrete example: please explain the difference between 子供が 遊んでいる and 子供は 遊んでいる. I am not a native, or anything even close, so if 遊んでいる is not in the right clause, please ignore it and concentrate on the が vs は part.

assistant: Wa (は) is a topic marker and ga (が) is a subject marker. Wa (は) is used to mark something that has already been introduced to the conversation and is familiar with both the speaker and the listener. Ga (が) is used when something is newly introduced into the conversation.

prompter: Could you provide a couple of example sentences, written in Japanese with the English translation in parentheses please?

assistant: I'm glad to do that! In the following, two example sentences in Japanese with their English translations:

彼女は先生です (Kanojo wa sensei desu) - She is a teacher.
私は学生です (Watashi wa gakusei desu) - I am a student.

Do you need more examples or have other questions related to Japan or the Japanese language?

prompter: From a strictly technical perspective is Japanese an efficient language?

assistant: Sure! Here are some common phrases in Japanese with English translations. These will be in Japanese characters and will require an understanding of the characters to accurately read.

こんにちは、はじめまして。(Hello, good to meet you.)
どこに行きたいですか? (Where would you like to go?)
私は多くの言語を理解できるチャットボットです! (I am a chat bot that can understand many languages!)

prompter: What are some similarities between the Japanese alphabet and the English alphabet?

assistant: Here are two example sentences from the start of a fairy tale. Note how the hunter is first introduced with ga (が), and in the second sentence ga (が) is replaced with wa (は). 

昔々、狩人が住んでいました。(Once a upon a time, there lived a hunter.)
狩人は、毎朝山を登りました。(Every morning the hunter climbed up the mountain.)

N.b. unlike in these two sentences the difference is not always akin to the English distinction between "a hunter" and "the hunter".

prompter: Can you tell me a story that begins with 昔々、狩人が住んでいました