Is there any justification for using “is” at the end of an English sentence, or is there a rule that forbids this?
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In other words: no, there is no such rule.
Here are a few sample sentences illustrating only some of the many possible constructions:
That's just how it is.
Go where the love is.
I don't know who David Pollock is.
I'm not affected. I don't know if anybody else is.
... then we can reduce the prison population by a third or a quarter or a half or whatever the target is.
Treat it like the delicate body part it is.
That's not unusual but getting up three or four times is.
You can say it's a political gift for us, and it is.
The farther right a category appears, the more valuable it is.
And then I looked up. Sheer delight. Heaven on Earth. Southern heaven, that is.
As you can see, most of these you can't even reword in an unawkward way, if at all.
However, what you can do to many of these constructions is use them with any other verb.
- That's just how it smells.
- Go where the love resides.
- I wonder how much David Pollock weighs.
- I don't feel affected. I don't know if anybody else does.
- You can say it's a political gift for us, and it shows.
- The farther right a category is, the more valuable it appears.
- The more you read, the wiser you become.
- Sheer delight. Heaven on Earth. Southern heaven, to wit.
In other words, it is perfectly normal for an English sentence to end in a verb, so you'd need to make a rather strong case why to be should be any different.
Note that most "rules" you may have heard in grade school actually aren't. I've heard tales of a study of supposed English spelling and grammar rules that found that the majority of them had more exceptions than non-exceptions.
The supposed i-before-e spelling "rule" with a gazillion competing exceptions is a personal hobby horse. I think most English speakers just memorize ie words, then walk around telling people there's a simple rule they are applying, when in fact there isn't.
I think it comes down to being seen as a lazy style of writing. For example, in the comments above, you could argue that they are incomplete, and should be:
Myself, I don't see what the problem is with using "is" to finish a sentence.
I don't know who's crazier, you or your uncle Bob, but I think that your uncle Bob is the craziest.
Ending with 'is' makes it feel like there's more to come and you didn't bother completing the sentence. So it's not forbidden, but you could certainly restructure the sentence to be more satisfying grammatically.