The plan has been approved = The plan was approved by someone (maybe we don’t know the exact name, or it is not required to give the name)
The plan is approved = This tells only the state/status - whether the plan is in approved state or not.
The plan was approved on Monday = Use ‘was’ whenever we want to give the exact time/day for that action.
If you’re trying to use ‘approve’ as a verb and insist on using ‘yet’, only the first sentence is correct. The difference is the probability of being approved of the budget. The first might while the second not.
It’s not a usual tense choice. It would only be used if the train had stayed at the station when it should have left over a number of occasions.
Why has the train been staying at the station (instead of leaving at 7pm each day)?
There are no set possible answers. Some examples might be:
It hasn’t been able to move.
It has had technical difficulties.
There have been problems with the engine.
For this question: “Why is the train staying at the station?”
is the answer “It isn’t approved to leave yet.” wrong?
Do you say, one can answer as, “It isn’t permitted to leave yet.”?
Please confirm.
It’s not wrong, but I can’t imagine a NES using ‘approved’ over ‘permitted’ in the context of trains and stations. In fact, I don’t think ‘staying at’ is the most likely term you would hear either. I think a more natural sounding question would be:
Why is the train still at the station?