がwho or what; the subject marker
が points out who or what: who did it, what is there, what you like. The word in front of it is news to the person listening.
How to form it
Noun + が
Likes, skills, wants
person は + thing が + …
With 好き, 上手, わかる, ほしい and できる, the thing takes が, not を.
The idea
With は, you name something the other person can already place, and the news comes after. With が, the word in front is news: the answer to a question, something you have just noticed, the thing you like.
Asking or answering "who?", "what?" or "which one?"
This is the one.
Ms. Tanaka came.
Something you have just noticed, or something that has just happened
This is what is happening.
It is snowing.
With 好き, 上手, わかる, ほしい
This is the thing you like, are good at, understand or want.
My father likes fish.
Who? Which one?
When you ask who or which one, the question word takes が, never は. The answer takes が too.
Which one would you like?
I'd like this one.
Choosing. The one you pick is the news.
Not said: どれは
(Which one would you like?)
Someone asks だれが行きますか, "Who is going?" Answering with は is a common slip.
Sounds odd: 私は
I'm going.
A different message: "I'm going, whatever the others do."
The same が is how you volunteer.
Something you have just noticed
You see or hear something and say it. You weren't talking about it before, so the whole sentence is news, the word in front of が included.
Oh, the bus is here.
The door opened.
The wind is strong.
Saying what is somewhere with ある or いる works the same way. Once the other person knows which one you mean, は takes over.
Oh, there's a cat over there.
That cat comes every day.
は or が?
Both can follow the same word. What changes is which part is news.
I am Tanaka.
Saying who you are. The news is your name.
I am Tanaka.
Someone is looking for Tanaka. The news is that it is you.
To tell someone about yourself, it is 私は, or more often no 私 at all: 田中です. 私が says "I'm the one".
This cake is the good one.
Picking it out from the others.
Oh, my father is here.
You have just seen him arrive.
My father is a doctor.
You are telling someone about him.
Reporting what you have just noticed takes が. Telling someone about a person or a thing takes は, even the first time you mention them.
A quick test: what question does your sentence answer? If the answer is the word in front of the particle, it is が. If the answer comes after, it is は. If the whole sentence answers "what's happening?", it is が again.
What you like takes が
English says "I like history", so を feels right. But 好き isn't a verb. It is a な-adjective that describes the thing, closer to "is liked", and the thing described takes が.
I study history.
Studying is something you do to it.
Not said: 私は
(I like history.)
Being an adjective, it changes like one: 好きじゃありません (don't like), 好きでした (liked), 好きな音楽 (music I like).
These everyday words take が the same way. None of them is something you do to a thing: わかる is closer to "is clear", できる to "is possible".
が好き・大好き・嫌いlike, love, dislike
What kind of music do you like?
が上手・下手good at, bad at
My older sister is good at tennis.
上手 is praise for someone else. About your own skill it sounds like boasting.
が得意・苦手strong at, weak at
I'm not good at cooking.
What people say about their own strong and weak points. 下手 about yourself is fine too.
がわかるunderstand
Do you understand the meaning of this kanji?
がほしいwant
I want new shoes.
For what you want yourself, or to ask what the other person wants. About anyone else it is a different form, ほしがっています.
In a "no" answer, or when you set one thing against another, this が often becomes は.
I don't like fish.
Close neighbours
はN5
Names what you are talking about, like "as for…". Also sets one thing against another.
Ms. Sasaki will come at eight.
が・けどN5
After a whole sentence, が is a different word. It means "but", and it also leads into a question: すみませんが….
I want to go, but I don't have time.
〜は〜が〜N5
Someone or something with は, then one part or feature of them with が. It is how you say someone is tall, has long hair or has a headache.
My older brother is tall.
Check yourself
1Someone asks this. Which answer fits?
だれが
Choose an answer to see why.
2"I like Japanese movies."
日本の
Choose an answer to see why.
3You are meeting someone for the first time. Which do you say?
Choose an answer to see why.
More detail
Inside a longer sentence
In the part of a sentence that describes a noun or gives a time, the one who does it takes が, not は.
This is a cake my mother made.
When my friend came, I was asleep.
More sentences
Which bus goes to the airport?
This bus goes to the airport.
Which seat is available?
There is a small cat in the garden.
A new shop opened in front of the station.
The light went out.
My older brother took this photograph.
My younger brother dislikes math.
I am not good at singing.
Maria understands Japanese well.
I am afraid of big dogs.