- Everything will be done by Tuesday.
- Everything will have been done by Tuesday.
What is the difference (semantic, grammatical), if any, between the two?
My question is different from this one because it features a different preposition, i.e. 'by' rather than 'at'.
The difference between "will have been done" and "will be done" is the same as the difference between "will have done" and "will do". The difference is active versus passive.
1) The work will have been done by Tuesday (by John): Passive
The active form would be: John will have done the work by Monday.
2) The work will be done by Monday (by John): Passive
The active form: John will do the work by Monday.
The first (will have been done) is an example of a future perfect tense. It is used to "to describe an event that is expected or planned to happen before a time of reference in the future" (Wikipedia)
The second is the "future" using will in a passive construction. It is used for predictions, statements of fact and intentions. Modern linguistics calls this a modal auxiliary. Here's a pretty good overview of its use: What's will?
In order to explain these passive constructions, one has to introduce an agent who need not be explicitly given when writing or saying these sentences.
The difference between "will be done" and "will have been done" is a relatively simple one. On is the future, and the other is past-in-future, future perfect (both are passive). The use of by, however, makes the distinction very murky.
If you specify a single point in time, doing something by that point is clear - it will be completely done. Thus, "it will be done by 3:00pm on Friday" means that it will be done, complete, nothing left to do as regards it, at or before 3:00pm on Friday. There is then no distinction between that and "it will have been done by 3:00pm on Friday", in terms of literal meaning.
Now, doing something by a certain day is ambiguous. Legal documents or regulations often specify what it means. One set of rules I've worked with specifies that "by a specified day" means "by 5:00pm on that day". Without that clarification, there might be local or occupational conventions, but it is hard to be entirely sure what a person means by it. I do not feel, however, that this changes the distinction (or lack thereof) between your examples - unless there's some local convention about that distinction as well.
Thus, while there is a grammatical distinction between your examples, one being in the future and the other future perfect, the fact that by is involved removes any essential semantic difference. However, I would say there is a shade of meaning distinction, that the basic future means that it might have been done very shortly before Tuesday, while the future perfect carries the implication that it will have been done in good time. That is quite possibly a dialectal and culturally-dependent shade of meaning, however.
