が
が is the subject marker. It is sometimes interchangeable with the topic marker は.
Topics and subjects
The differences between は and が look kind of like this:
- Bobはぼくのともだちだ。 ピザがほしい
(Literal direct translation) Bob. (He) is my friend. Pizza want.
Bob is my friend. (He) wants pizza.
It’s the last sentence I want you to notice. There is no topic mentioned, but there is a subject; pizza. However, it makes no sense without knowing that it’s bob who wants the pizza.
Generally, は sets aside something as a topic, and subsequent sentences and subjects relate to the topic. (until changed).
が for specificity
が is preferred over は when you have a narrow, specific target, especially when it already relates to a topic. が tends to place more emphasis on the subject
Using が puts emphasis on the subject of the sentence.
が must be used when the words modifying the subject are specific to a particular individual or group.
In the following sentences, が is generally preferred:
- ねこはこわい – Cats are scary (normally thought of as scary, i.e. not necessarily to me).
- (私は)ねこがこわい – Cats are scary (to me)
- かれはじょうず – He is good (skillful)
- かれがじょうず – He is good (skillful)
Some words, like adjectives and states, are by their nature specific:
- (私は)猫が好き – I like cat(s). (i.e. like is specific to cat; for me)
- ピアノがじょうず – Good at the piano. (“good” is specific to pianos)
You may still use は with these words only if you intend to make the subject the topic, and there is no other topic (like I).
”が” words
Other words that will typically use が due to being specific to a thing.
- すき (liked)
- きらい (hated)
- こわい (scary)
- じょうず (skillful)
- へた (unskillful)
- Question words
が (New topic)
Lastly, when there is a topic, が can be used to change the topic.
A: 彼はすごい。- He’s amazing.
B:そうだけど、彼女がもっとすごい。Sure, but she’s more amazing.
For an attempt at a more in depth look at the difference between は and が see here.