Syntax: -te form compound verb. Uses intransitive verbs.
Usage: Shows a state that results from a deliberate action.
Explanation: While あるby itself shows that something exists, when it is used after the -te form of a transitive verb it shows what somebody did to it to get it into the state it is in now. This is in contrast to いる, which has a similar structure, but is used with intransitive verbs and shows what happened, rather than what was done.
The window has been opened
Note that even though the verb is transitive, が not を is used because the particle agrees with the final ある, not the -te form before it.