〜てある te aru
〜てある（te aru）means “has been done (and remains so); is in a prepared state”. Made from a transitive verb in て-form + ある. Formation: V（て形・他動詞）+ ある
has been done (and remains so); is in a prepared state
N4Formation
V（て形・他動詞）+ ある
Explanation
Made from a transitive verb in て-form + ある. It expresses a resulting state that someone created on purpose and that still remains — the focus is on the state, not the action. The thing acted on is usually marked with が.
Examples
Note
Contrast with 〜ている (state from an intransitive verb): てある stresses someone's intention; 窓が開けてある (someone opened it on purpose) vs 窓が開いている (it's simply open).
FAQ
〜てある — what does it mean and how is it used?
has been done (and remains so); is in a prepared state. Made from a transitive verb in て-form + ある. It expresses a resulting state that someone created on purpose and that still remains — the focus is on the state, not the action. The thing acted on is usually marked with が.
〜てある — how do you form it?
V（て形・他動詞）+ ある
〜てある — any usage notes?
Contrast with 〜ている (state from an intransitive verb): てある stresses someone's intention; 窓が開けてある (someone opened it on purpose) vs 窓が開いている (it's simply open).