- She won't be understanding it
- She won't have been understanding it
- She might not be understanding it
- She might not have been understanding it
As far as I know 'understand' is a stative verb so -ing is ungrammatical and unidiomatic as well.
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The -ing form would be unusual with understand, but I wouldn't go so far as to say that it is ungrammatical or unidiomatic in all circumstances.
You would be drawing special attention to the ongoing act of comprehension if you were to use the -ing form with understand, and so you would simply need a context that supported such emphasis.
As you engage a person with aphasia in conversation, be aware that although they might be understanding your words as you say them, they may be unable to repeat those words back to you.
Like TRomano said, you don't usually find "to understand" used this way. Understanding usually isn't referred to as an ongoing even with the -ing ending. I think that, as you said, it's a stative verb.
Here's some perspective as a native speaker that might help illustrate how to use "to understand" more naturally
If you want to get across that someone is currently confused, or will be confused, etc., you can simply say
She won't understand it when we explain it to her
She might not have understood us, we should explain it more clearly next time
To look at it a different way, let's say for example that you're in the middle of a conversation, and you are explaining something to a friend, and they look confused. You might think to yourself that this confusion is an ongoing event, and that -ing is an appropriate ending, like
"I'm eating" = "I'm (in the process of) eating (right now)"
"He's not understanding" = "He's not understanding (right now)"
But with "to understand", this situation would simply be described with
He doesn't understand
or if you want to see an example of using an -ing ending, you might say something like
He's becoming confused / He's getting confused
Where "to get" in this example is a colloquial way to say "to become". In this example "to become" and "to get" aren't stative verbs I believe, and so they can be used this way. (Or rather, because they are used this way, they are not called stative verbs)
I will say though, that if you did say "She might not have been understanding it" to a native speaker they will have no problem understanding what you mean.