"Hvis det er" and "Hvis det er tilfældet"

[Ludwig Lukasiewicz]

I often hear people say "Hvis det er" in the end of their expression, but I am not really sure about what they exactly mean by this phrase, since there is no following content. The expression seems like "Du kan XXX, hvis det er.".

I also hear people sy "Hvis det er tilfældet, så..." as a kind of expression of hypothesis, but is the hypothesis about the past or the future, or can be used for both?
For example, "Hvis det er tilfældet, så kan vi ikke åbne døren." is about the hypothesis in the past "If I have lost my key (in the past), then we cannot open the door (now)."
"Hvis det er tilfældet, så kommer jeg og hjælper dig" is about a hypothesis in the future "If it happens (in the future), I will come and help you."
To which kind of hypothesis can I use "hvis det er tilfældet"? or both to the past and the future?
Besides, is the "tilfældet" in such expression only used as something negative? Can I also use it for some positive thing, such as "If you buy me a bunch of flowers, I will be happy."?

 

[Wehage]

In your example, "du kan XXX, hvis det er," I most commonly hear it used as a set phrase abbreviated from "hvis det er det, du vil," ("if that is what you want") and not "hvis det er tilfældet, at XXX." I would argue that it is interchangeable with "du kan XXX, hvis du vil."

"Hvis det er tilfældet" is interchangeable with "if that is the case," and is neutral rather than positive or negative. You could say "hvis det er tilfældet, at du har købt blomster til mig" or "hvis det er tilfældet, at du har dræbt min kanin," but as this particular adverbial clause is rather formal, it seems unidiomatic to me in use with emotionally loaded conversations (except perhaps in a court trial). Even in its present tense, the phrase can refer to past, present, and future:

Hvis det er tilfældet, at han kom for sent, må vi tage en snak med ham. (Past)

Hvis det er tilfældet, at Jorden er flad, så lever Elvis på mit loft. (Present)

Hvis det er tilfældet, at Donald Trump bliver genvalgt, så flytter jeg til Canada. (Future)

If you change "er" to "var," the phrase becomes a hypothetical (imaginary scenario, unlikely):

Hvis det var tilfældet, havde jeg ikke gjort det.

I hope this helps.

 

[Emil100]

Also, "hvis det er", in the end of an expression, can often be understood as "if it is/becomes relevant".
Whether "hvis det er" means "if that is what you want" (as explained by Wehage above) or "if it is/becomes relevant" depends on the specific context in question.

Besides, is the "tilfældet" in such expression only used as something negative? Can I also use it for some positive thing, such as "If you buy me a bunch of flowers, I will be happy."?

The use of "hvis det er tilfældet" for a positive thing such as in the example with flowers would not be wrong in terms of meaning. But I would say that it would read odd or have a little negative effect because "hvis det er tilfældet" would be a little too formal language for such use.