I �ll take your word about your last statement, since I can �t prove that the sentence doesn �t exist anywhere in the English-speaking word. 

Bruce 

Bruce, this is what I said :

 

"But, if you use a phrase that clearly implies the future, you can also use the perfect infinitive for the future, just like you would use the future perfect tense "

 

As for the passive, I am not sure that the change of voice will make the sentence lose its correctness, though ...

 

Are you saying that while this sentence is OK 

 

Then its passive variant is wrong?

 

 

Interesting.  

 

I agree that this sentence is fine: By that date all the forms required must have been completed and returned to the School Administration office.

 

Why then does �It must have been finished by next Monday � seem so wrong then?  One possible reason is that, to the native speaker, �finished � seems like an adjective in that sentence rather than a past participle.  The verb �complete � is a transitive verb, and, with the object of �complete � stated in the sentence (in the passive voice), this sentence is clearly the present perfect in the passive voice.   �Finish, � on the other hand, is both an intransitive and a transitive verb.  I would argue that, without a clear object stated, native speakers perceive �finish � as an intransitive verb.   �The movie finishes at 9 p.m. �  and �What time does the dance finish? � can be simplified as �It finishes at 9 p.m. � or �What time does it finish? �  In these cases �it � is the subject, not the object.  In the sentence �It must have been finished by next Monday, � we see (or hear) �it � as the subject, not the object.  Thus �it must have been finished � is not perceived as being in the passive voice, because �it � is interpreted to be the subject rather than the object. Therefore we interpret �It must have been finished � to be the past tense of �must � showing deduction -- �It must have been raining -- that �s why his shoes were wet. �   �Their marriage must have been over by then because he went on holiday by himself. �  And, since �must have � is a past tense form, it is clearly impossible/wrong to use �by next Monday � with it.  

 

In your above sentence, it �s also possible that �must have been � is used instead of �must be � in order to make a slight distinction in the time of the two different actions (completing the forms and returning them).  First, they �re completed.  Then, they �re returned.  The �be � before �returned � is omitted as something that is understood. 

 

If you can change the sentence around and add other elements to it to make it clear that �finish � is being used as a transitive verb, then, yeah, maybe it �s possible.  This might be one example.  The teacher, talking to his students, says:

 

�Okay, you �ve had a lot of homework to do these past few months.  I know it �s been a difficult week, but, by next Monday, all of your assignments must have been finished and then put on my desk. �  Personally, as a Canadian, I would probably say �... must be finished � in that sentence but I know in the U.K. the perfect tense is used more often than it is in North America.

Also, I guess I misspoke before. It was your examples that were passive, not your statement.. 

 

Bruce