I Wouldn't Say That

Example 1:
Ron: Hey Sally, let's see a movie at the theater tonight.
Sally: That's a great idea! What should we see?
Ron: I really want to see the new "Rambo" movie! It looks awesome!
Sally: Hmm, could we see that movie another day? I don't want to watch an action movie.
Ron: Oh, OK. I didn't know you hated action movies.
Sally: I wouldn't say that. Tonight just seems like a good night for a comedy. 

Example 2:
Student 1: More homework?! This teacher isn't fair. I think she hates us.
Student 2: I wouldn't say that. I think she just wants us to practice as much as we can. 

Explanation:
I wouldn't say that is a very polite way to disagree with someone's comment. You can add a comment to explain why you disagree. 
In Example 1, Ron thinks Sally hates action movies. Sally politely disagrees with him and explains that she would rather see a comedy movie. 
Ron: Oh, OK. I didn't know you hated action movies. (Sally disagrees with this)
Sally: I wouldn't say that. Tonight just seems like a good night for a comedy. (Sally explains why she disagrees)
In Example 2, Student 1 thinks the teacher is too strict because she assigns a lot of homework. Student 2 disagrees and offers a different point of view.
This idiom can be found in LSI's textbook titled "Speaking Transitions"  This book is used at LSI to teach the Level 4 Speaking class.  For more informatioin please visit idiomstudy.blogspot.com