～てあるSomething is done, Has been done, Remaining state

Like 'verb + ている', 'verb + てある' is a structure that will convey an ongoing state. However, 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).

As 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.

Whether the sentence uses が or を to mark the object will depend on what is being emphasized. In cases where を is used, the emphasis is on the whoever performed the action, regardless of whether they are strictly mentioned in the sentence. However, が focuses more on the ongoing state of the object, without any emphasis being placed on the person who may have performed that action originally. Additionally, を may mark the object in sentences where the action itself involves some kind of preparation. For example:

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), が will be preferred when marking the object. This 'preparation' can be thought of as a time investment, where the invested time of the doer leads to the sentence more naturally expressing that person as the subject, and the object as what will take を.