ています vs. てあります: What's the real difference?

First, you must know what transitive and intransitive verbs in Japanese are. If you are unsure about the difference, please check the link below.

https://www.livinginjapanasaforeigner.com/2022/08/the-confusing-part-of-japanese.html

The difference between ています and てあります

1. As transitive verbs, the subject of the sentence is different.

(1) "てある (てあります)"  Only transitional verbs have this form. It is used to emphasize the result. The subject is before "は."

For example:

「窓は開けてあります。」　　[Emphasize that (the window) is open.]

「食器は洗ってあります。」　　[Emphasize that (the tableware) is finished.]

「テレビは直してあります。」 　[Emphasize that (the TV) is fixed).]

(2) "ている (ています)" stressed that the action was ongoing. The subject is omitted.

For example:

「窓を開けています。」　　[Emphasize (the omitted subject) is opening the window.]

「食器を洗っています。」　　　[Emphasize someone is washing the dishes.]

「テレビを修理しています。」　　[Emphasis some is repairing the TV.]

2. As intransitive verbs, the form is different.

Intransitive verbs only have the form of "ている (ています)," and there is no such form of "てある (てあります)." Its characteristic is that there is no object in the sentence here. This is different from transitive verbs. 

(1) In a continuous verb sentence, the action is continuing. The motion is in progress.

For example:

「子供が泣いています。」　The child is crying.

「犬が走っています。」　The dog is running.

「雨が降っています。」　It's raining.

(2) In the instantaneous verb sentence, the result of the action is continuous. That is, it represents a state.

For example:

「部屋の窓は、開いています。」[The result of "開く" is (that the window of the room is always open)]

「ストーブが付いています。」[The development of "付く" is (the stove keeps burning)]

Why should we distinguish them?

Well, as I said at the beginning.
You must check what transitive&intransitive verbs are in Japanese. Only in this way can you really understand why. 

But I'd better explain it briefly here!

The most significant difference between these two verbs is whether someone must participate in the action. 
For example, you are the only person who is involved in the action of getting up, so "起きる" is an intransitive verb; When someone is trying to wake you up, this is a transitive verb. We call this action "起こす" in Japanese.

Therefore, you will find that in English, there are two phrases.

And these two phrases, "get up&wake sb. up," have completely different meanings, but in Japanese, they use the same Kanji to form words to represent these two phrases.. (起きる&起こす)

In Conclusion

We have three ways in Japanese to indicate that the window is open.

• 窓は開けてあります。
• 窓を開けています。
• 窓は開いています。

So you may be confused about why there are three kinds.

The first is to emphasize the result, and the subject is the window. (Result ONLY)

The second stresses the window's status, and the subject is who opened the window. (I am doing this!)

The third is representing a state. It only has subjects, no objects. (Just stating a fact.)

The first two sentences usually have subjects and objects because they use transitive verbs. 

But the third sentence uses an intransitive verb, so there should be only a subject in the sentence.

But you may ask why I gave four examples. 

The reason is simple.

There are instantaneous and continuous actions in intransitive verbs in Japanese.

And "開く" this action is instantaneous. (intransitive verbs)

"走る,""降る," and "泣く" These three actions will not stop soon once they start, so they are continuous verbs. (intransitive verbs)

When an instantaneous verb adds a "ています," "that means the result. (intransitive verbs)

When a continuous verb adds a "ています," that means the action is ongoing. (intransitive verbs)

Transitive verbs have no distinction between instantaneous action and continuous action.

What do I think?

I don't think you need to remember "てあります" at all because it is not often used.

We have many ways to express that something is done in Japanese. It is not necessary to use "てあります." You can just say "窓は開けた.," But if you learned this way, you would be more like a native.

My conclusion

When you use "ています" in intransitive verb sentences, you will naturally know whether the action is ongoing or done (result).

When it comes to transitive verbs, it's easy for you to think it's about telling who is doing what.

This is just my personal idea:

"てあります" in most cases, looks like you follow someone's command and then tell the commander that you have completed it.