I'm [suffix] to [prefix] it, [infix] it's [whole].
The above sentence, when the affix gaps are filled, will be a logical statement. What is the statement?
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Sign up to join this communityA grammatically correct statement which fits here - and acts in a pleasingly self-confirmatory way - is:
I'm _ABLE to UNDERSTAND_ it, _AND_ it's UNDERSTANDABLE!
It's a pleasing fit for the puzzle, since its meaning is perfectly appropriate here!
Alternative Answer
Context
Somebody is on their way to their own trial when they get stuck in traffic. They get a phonecall from a friend who asks how the trial is going.
Response
I'm STILL to STAND it, AND it's STANDSTILL.
I'm _ABLE to POST_ it, _MARK_ it's POSTMARKABLE.
Everyday American English translation: "I'm able to mail it, notice it's able to be postmarked."
"Post", when used as a verb, can mean to "mail" something (i.e. send an item in the mail). Example: I'm going to post a letter to my mother.
"Mark", when used as a verb, can mean to "notice" something (i.e. pay special attention to something). Example: Mark my words, she'll always remember this day!
"Postmark", when used as a verb, refers to the action of putting a postmark (a stamp or sticker showing the post office and date of mailing) on something. Example: The letter was postmarked on January 3rd.
Scenario: A customer is trying to mail a package that their child has decorated, but the postal worker can't see where she can put the postmark, and says "I'm sorry, you can't send this package, I'm not able to postmark it." The customer points out the one corner of the package free of decoration and says, "I'm able to post it, mark it's postmarkable."
Here's a second solution inspired by this first one:
I'm _ABLE to REMARK_ it, _MARK_ it's REMARKABLE.
Example scenario: Someone appreciating a contested work of art, and encouraging others to take note of it. "This piece of art doesn't seem noteworthy. I don't see the beauty in it... do you?" "I'm able to remark it, mark it's remarkable." "We'll see..."
And a third solution inspired by @hexomino's solution:
I'm _UP to STAND_ it, _AND_ it's STANDUP!
Example scenario: Someone going to stand trial (and they believe it is the right thing to do), being questioned: "Are you sure you're up for standing trial?" "I'm up to stand it, and it's standup."