(I think I'm going to open a can of worms with this answer but I've done some research so, don't blame me.)
In the student's text book, New English File Upper-Intermediate Oxford University press
Page 138 it says:
The opposite of "must have" is "can't have" NOT "mustn't have"
So for some it is considered standard English to use: can't have or couldn't have instead of mustn't have when you are speculating or guessing about the past in questions and negative sentences.
He couldn't have been hungry
means practically the same as
He can't have been hungry
They both express a strong conviction in the past, the speaker can choose to add further information in order to back up his claim.
A: John didn't eat his cereal this morning.
B: He can't/couldn't have been hungry. He usually has breakfast.
Thus the speaker is saying it's impossible that John was hungry because he knows John never leaves home without eating something. Must not (mustn't) means something quite different, you are forbidding someone or something from performing an action now, in the present and it is not used for speculating in the past.
On p394 in Practical English Usage by Michael Swan:
Must is used with the perfect infinitive for deductions about the past.
- "The lights have gone out" -- "A fuse must have blown."
- "We went to Majorca." -- "That must have been nice."
Must is only used in this way in affirmative sentences. In questions and negatives, we use can and can't instead.
This is also confirmed by A Practical English Grammar by A.J.Thomson A.V. Martinet 4th edition on page 148.
Let's look at a few more sentences in the same tense (present perfect) as your first example:
Mary has eaten the cake.
I have finished the report.
Someone has taken my phone.
In each case the first phrase of the sentence is the doer of the action (Mary does the eating, I do the finishing, someone does the taking.)
From this it is clear that your first example does not make sense (although it is grammatically correct). An idea cannot do the deleting. It does make sense, however, to say: Someone has deleted the idea (actually, deleted the file would be a better example).
Your second sentence, on the other hand, is both grammatical and makes sense. It is in the passive form of the same tense. If we convert the examples above to the passive, then we get:
The cake has been eaten (by Mary).
The report has been finished (by me).
My phone has been taken (by someone).
Your second sentence fits in here:
The file has been deleted (by somebody).
We use the passive like this when we want to shift the focus of the sentence away from the doer of the action. Maybe we don't know who did the action, or it is obvious, or we don't care who did it. The passive allows us to focus on what happened and does not require us to mention the doer.
Best Answer
This is referring to an event that occurred in the past. An event happened and its effectiveness was sup-par. For example:
As for your other phrase:
This contains the possibility of future effectiveness. A demonstration of the effectiveness has obviously been conducted, and the speaker has decided that improvements in the process could be made so that it is more effective in the future. For example: