Grammar JLPT Level N4 - Lesson 17

Like 'verb + ている', 'verb + てある ' is a structure that will convey an ongoing state. However, that is where the similarities stop. てある may only be used with transitive verbs, and highlights the object itself as being the subject of the sentence (rather than the initial person/thing that performed the action).
Because ある is being used, it highlights the object as being inanimate (or unable to undo the action that was performed on it). Due to this, てある is regularly translated as 'to be left (A)', or 'to remain (A)', where (A) is the original verb that somebody performed on the object.

• あなたの弁当(べんとう)は机(つくえ)の上(うえ)に置(お)いてあるよ。
  I have left you lunch on the table. (Literally 'your lunch is in the state of having been left on the table')
• 車(くるま)は駐車場(ちゅうしゃじょう)の真(ま)ん中(なか)に止(と)めてある 。
  I parked the car in the middle of the parking lot. (Literally 'your car is in the state of having been stopped in the carpark')
• 庭(にわ)にトウモロコシが植(う)えてある から気(き)を付(つ)けてね。
  Corn has been planted in the yard, so please be careful. (Literally 'corn is in the state of having been planted')

が may mark the object in these sentences when the action itself involves some kind of preparation. Examples are as follows:
予約(よやく) - Reservation (requires time investment to reserve)
準備(じゅんび) - Preparation (requires time investment to prepare)
録音(ろくおん) - Recording (requires time investment to make the recording)
In situations that involve no real preparation (opening a door, for example), を will be preferred when marking the object.