Inference vs must be true are quite similar concepts. But "must be true" is something that definitely cannot have a false side to it - otherwise it violates "must" be true.
Inference - this is a statement (usually among the answer choices) that can be supported by information in the passage. Thus, somewhere in the passage, we have a supporting statement or phrase that would support the "Inferred" statement. Because it is supported by something in the passage, it is called an "inference".
An inference is not necessarily something that *must* be true. Here, you deal with something that can be supported by information in the passage, but might not be a true statement ALL the time. So the key point to pay attention to is that *must* be true has absolutely no exceptions - whereas an inference is more flexible and *only* requires some support from somewhere in the passage.