がある

がある says that a thing is there: "there is", "there are". It follows the noun for the thing, and the place comes first with に. Polite speech uses があります.

ある or いる

ある is for things that don't move by themselves: objects, plants, places, ideas. People and animals take いる.

• × 庭に犬がある → 庭に犬がいる

Having something

With the owner as topic, がある means "have". The owner takes は or には, and the thing keeps が.

Not there, was there

The negative of ある is ない, not ×あらない. The polite forms are ありません, ありました and ありませんでした.

Events

がある also says that an event takes place: a test, a party, an earthquake. The place where an event happens takes で, not に.

• × 公園にお祭りがある → 公園でお祭りがある