〜てある
A state remaining after a deliberate actionmachine translation
FormationTransitive verb て-form + ある
〜てある expresses the state remaining after someone has deliberately done something: 窓が開けてある (the window has been left open — because someone opened it in preparation for something). This pattern always goes with a transitive verb, and the affected object usually takes the particle が.
The easiest point of confusion is with 〜ている. Compare: 窓が開いている (intransitive 開く — the window is open, merely describing a state, saying nothing about who opened it) and 窓が開けてある (transitive 開ける — the window has been left open, definitely by human hands and with a purpose behind it). Whenever you see 〜てある, read into it that "someone did this deliberately".
〜てある is the natural result of 〜ておく: the act of preparing uses 〜ておく, while the state afterward uses 〜てある — ビールを買っておきました (I bought beer in advance) → ビールが買ってあります (the beer has already been bought).
Examples
暑いので、窓が開けてあります。
Since it's hot, the windows have been left open.
机の上にメモが置いてあります。
There's a memo left on the desk.
レストランはもう予約してあります。
The restaurant has already been booked.