What is the meaning of following sentences?
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The work was to have been complete today.
(Can I also say: "The work was to have completed today" ? Or The work was to have been completed today" But why "been" has been used in above sentence?)
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If she were to have thought about it a bit more, she wouldn't have done that.
How and when we can use the sentence construction or structure was/were to + have + past participle or v3? Is it a causative form?
Best Answer
These constructions indicate hypothetical but not real situations.
Was to have
Was to have, as in your first sentence describes an action that was intended but never realised. In the case of sentence 1, someone planned to have the work completed today, but it wasn't completed; either because whoever was supposed to do it chose to do something else, or because it took longer than expected.
This sentence is valid both for after the fact (we see now that it wasn't done) or as soon as the speaker realises that the planned action is impossible (we see now that we have two days' worth of work left to do; clearly we won't be done today).
Also, was here agrees with its subject for number in all circumstances. If you had a plural subject, you might say something like We were to have finished the work today. Were to have (and were in general in this use) is more complicated.
Were to have
Were to have, or rather if [subject] were to have, indicates a hypothetical action that could have happened but now can't (or at least can't happen with the described effect) because the situation has passed. In the case of sentence 2, she has done the thing already, and now even if she thinks about it a bit more, it won't prevent her from already having done the thing.
If we replace were to have with simply were to, we get a sentence about something that could potentially happen with the described effect, though it probably won't. Sentence 2, modified this way, would look like this:
In this case, she still has the option to think about it a bit more and thus not do the thing. However, the phrasing with were suggests that we don't expect her to take this option - she can choose not to do the thing, but she probably will do it. This is in contrast with a simple if-statement without were, such as this:
With this sentence, we don't make any assumptions about whether or not she will think about it a bit more - she may or may not, we don't know.
There isn't much difference between if... were to have and simply if... had, in terms of how they describe a situation. These two sentences here describe exactly the same situation, in fact:
The difference here is one of formality and archaism - were to have is more archaic, and thus more formal, than simply had. Were here is one of the last remaining uses of English's subjunctive mood, a verb form used in Indo-European languages to indicate various kinds of unrealised situations. Many forms of colloquial speech have now lost the subjunctive entirely, and would prefer simply had over were to have.
This also means that it has strange plurality agreement sometimes, and may appear as were with singular or plural subjects - note that in your example, we have if she were to have, which otherwise would be wrong. Some speakers give it normal plurality agreement, though, and might say if she was to have here instead; I think I might use either were or was at random. The most formal and prescriptively 'correct' choice is were to have, but keep in mind that depending on your situation this may sound more stuck-up and snooty than you might intend - use it in writing, but probably don't say it to your friends. (You can replace simple had with were to have only in similar hypothetical situations with if - if I had gone and if I were to have gone are mostly the same, but he got it done before I had gone can't be transformed into *he got it done before I were to have gone, which is ungrammatical and meaningless.)
These aren't causative forms, though they do look a lot like the causative form have (someone) do (something) - if sentence 1 was causative, it would look like I had him complete the work today, which would simply mean that I asked him to do the work today and he did it. You can, if you want, make a sentence that is both hypothetical and causative by combining the hypothetical was to and the hypothetical have... do, which comes out as something such as I was to have had him complete the work today (though that's really quite awkward) - meaning that I intended to make him complete the work today, but I haven't had the chance to ask him. You can tell the difference in that have as a causative will always be followed by a noun or pronoun, and then a verb in its base uninflected form; while was to have and such will always be followed directly by a verb in its past participial form.
To answer your other specific question - you can theoretically say the work was to have completed today, but this complete is in some ways a different verb from the complete used in the work was to have been completed today. There are, in effect, two separate words that both look like complete:
There's a different connotation to sentences using the intransitive complete versus a passivised transitive complete (→ be completed). The passivised transitive version still implies that someone somewhere is acting to finish this thing, and that if we left it alone it would never be done. The intransitive version implies that the action is getting closer to being done all on its own, and it would still get done whether or not anyone was working on it. The work was to have been completed today sounds like someone was supposed to have been working on it, and it got left undone and didn't get finished. The work was to have completed today sounds like whatever thing we set up and left to run took longer than it was meant to (or stopped partway through and needed setting up again).