It's not a problem to rely on a powerful weapon. If you can find one, that is.
I don't get that "that is". Could somebody please explain it?
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
This is a very common expression in English that can mean all of the following: that is to say (that is is really just the short form of that is to say), in other words, or more exactly, or even of course. The Collins English Dictionary, the link to which I provided above, defines it as follows:
You use that is to say or that's to say to indicate that you are about to express the same idea more clearly or precisely.
So, we could rewrite your example like this:
It's not a problem to rely on a powerful weapon. If you can find one, of course.
It's not a problem to rely on a powerful weapon. That is to say, if you can find one.
Hopefully, things make more sense to you now. Again, this is an expression and as such it should be learned as a single unit.
In grammar, there's what is called "Ellipsis". It simply means, the deletion of parts of a sentence. For instance, "They said Richard is good but I don't think he is." Here, the word "good" has been deleted towards the end of the sentence. Also, we have "Substitution" which is the replacement on one part of a sentence with another. Ellipsis deletes while substitition replaces. An example of substitution is when you say "The man is kind. He is kind". "The man has been substituted with "He" in that sentence. That's the same thing happening here. The sentence says "It's not a problem to rely on a powerful weapon. If you can find one, that is. The complete sentence could have been "It's not a problem to rely on a powerful weapon if you can find a weapon that is powerful" which means it's no big deal to use a powerful weapon if you have one. In the original sentence, there are instances of Ellipsis and Substitution. "One" replaces a weapon and "powerful" is deleted. But the sentence made a full sense when the deleted parts were added.