[Supperman]

ga

subject maker

Ga marks what Japanese call the grammatical subject of a sentence. Think of the subject in the following two ways;

・First, in neutral descriptions of observable actions or situations
・Second, for special emphasis, to distinguish a particular person or thing from all others.
・When an interrogative pronoun is used as the subject of a sentence, Ga must always be used too.


object maker

O usually marks the object. But ga marks the object of certain non-action verbs and certain verbal adjectives and adjectival nouns.

Examples
Yoko san wa, eigo ga wakarimasu./ Yoko understands English.
wa - topic maker
eigo - English
ga - obj.maker
wakarimasu - understands

Nihongo ga dekimasu ka./ Can you speak Japanese?

Hon ga hoshii desu. / I want a book.

Meri san wa, tenisu ga jozu desu./ Mary is good at tennis.

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o

direct object maker

Examples

Senshu sono hon o yomimashita. /I read that book last week.

Jon san wa eiga o mimashita. /John saw a movie.

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So,
O marks the object of action verbs.
Ga marks the object of certain non-action verbs.