How to Use ～てある

Creative title, huh? But I don't know what other title I could use that's not several sentences, because that's how much is needed to describe the concept that's introduced in this chapter of Genki.

The て form of a verb followed by ある is used "to characterize a situation that has been brought about on purpose by somebody who remains unnamed in the sentence." (Thanks Genki, for both that and the following example.)

寒いので、ヒーターがつけてあります。
The heater is on, because it's cold.
(The heater was turned on and has been kept that way.)

At first glance, this looks awfully similar to ～ている form because it's describing the ongoing state of an object (i.e., ヒーターがついています。) But there's a couple differences between these two sentences.

With ～てある form, we're describing an action that was purposefully performed by someone (who's unnamed), and how the noun in the sentence is affected as a result of that. This form almost always uses transitive verbs.

With ～ている form, we're describing the ongoing state of a noun, without any reference to how it reached that state. So in my example in parentheses, I'm merely saying that the heater is on, but not expressing that someone turned it on and has kept it that way. This form almost always uses intransitive verbs.

Now, ～てある form can be used to express situations that are a result of  ～ておきました (which describes actions that are done in preparation), and can still be observed at this moment. Because the action is being observed at this moment, ある will remain in present tense. Let me give another Genki example.

予約をしておきました。
I made a reservation in advance.

レストランの予約がしてあります。
A restaurant reservation has been made.

The second sentence is a result of the first sentence. You've made a reservation (a preparatory action), which has led to the state of the reservation existing (described with てあります there).

Now for a few of my own practice sentences. Note that が is often used in place of を for the direct object with these transitive verbs.

外は暑いから、ドアが開けてある。
It's hot out, so the door is open.

試合で汚くになる服が洗ってある。
The clothes that got dirty at the match are washed.

アヤトはぺこぺこだから、おにぎりを料理してある。
Ayato's hungry, so there are rice balls cooking.

カナト要ったなっとが買ってある。
The natto that Kanato wanted has been bought.