Can someone explain to me what the grammar rule for "to be" is in this sentence:
"Landlords and tenants often disagree over paying for things to be mended".
Is it a passive construct? Here's where I found the sentence. https://ibb.co/cF931Mp
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This is actually a trickier question than it seems at first (and it actually took me a little bit of time to figure out what I think is the technically right answer to this). This use of "to be" is something that native English speakers are all pretty much familiar with, but does not seem to be discussed much in textbooks, etc.
This is actually a form of the future hypothetical passive. Let's break that down a bit:
To start with, the simple active form of the "things to be mended" in this case is actually:
(somebody) mends things
(That is somebody is fixing or repairing the things)
The simple passive of this, therefore, would be:
things are being mended
However, in this case, this is being used in a "for" clause expressing an expected use for the money being paid. Since this is only a potential outcome (things are not being mended right now, they would potentially be mended if it is actually paid for), it is appropriate to use a hypothetical construction. In many hypothetical statements this is done using the prefix "were", but since we already have a preposition ("for") introducing this clause, the "for" takes the place that "were" usually would:
were things being mended (simple hypothetical)
paying for things being mended ("for" clause form)
But in this case, this hypothetical situation isn't something that's (hypothetically) already happening (present tense), but the "paying" is happening in the present tense so that the mending can potentially happen at some later time (after the payment happens), so we also want to use the future tense, not the present. The way to do a future hypothetical passive is with "to be":
were things to be mended (simple hypothetical)
So the corresponding "for" clause ends up being:
paying for things to be mended
And there you go.
Now, it's also important to note (as came up in the comments) that there is a very similar construction, which is also sometimes used, but means something slightly different:
paying for the things to be mended
This looks like the same thing, (paying for somebody to mend "the things"), but actually, the addition of "the" in this case changes how the whole phrase is interpreted. In the form without "the", the target of the "for" is the whole phrase "things to be mended" as one (passive construction) unit: it is implying that what you're paying for is actually the mending, not the things. So it's basically:
paying for (things to be mended)
However, when you add "the", it changes the meaning to:
paying for (the things (to be mended))
That is, "to be mended" actually becomes a subordinate clause of "things" (telling you which things), but "the things" is now the actual target of "for", so you're paying for the things, not the mending.
(This might not seem like a big difference, but if you've got two $500 lamps which will cost $10 each to fix, then "paying for things to be mended" would likely only cost you $20, while "paying for the things to be mended" (e.g. "you break it, you buy it!") might be more like $1000)
Technically, one could argue that either sentence could be interpreted either way, but in reality the first form (without "the") is pretty much always interpreted as a passive construct (paying for the mending), while the second form (with "the") is always interpreted as a subordinate clause (paying for the things). To be honest, I can't really give you a good reason why that is the case, though. As far as I can tell, it's just one of those things "everybody knows"..