N4
てある
Something is done, Has been done, Remaining state
庭にトウモロコシが植えてあるから気を付けてね。
Corn has been planted in the yard, so be careful.
Explanation
「transitive verb+てある」 says someone did something on purpose and the result is still there: 「カレンダーに予定が書いてある」 (The schedule is written on the calendar), 「壁に絵が掛けてある」 (A picture is hung on the wall).
Three points: the verb must be transitive; the original 「を」 usually becomes 「が」; it does not say who did it, but it feels like someone did it deliberately.
Mixed up with
「窓が開けてある」 (someone left the window open on purpose) is a result of human action. 「窓が開いている」 (the window is open) just describes the state, with no thought of who or why.
When answering whether something is ready, 「を」 can stay: 「切符はもう買ってあります」 (I have already bought the tickets).