I'm a grizzled veteran of hundreds of critiques, and one thing you master is saying negatives without attacking the author.
Stick to the text. You don't say "Ms. Smith fails to explain any reason for Elliott's changed behavior." Instead, you say, "The novel fails to explain any reason for Elliott's changed behavior."
Accept partial blame for a book's flaws and faults if you can. "This reader could not discern any reason for Elliott's changed behavior."
Don't rewrite it as if it were your book. "If Elliott had been a secret admirer, the change in his behavior would make sense."
Cushion your negatives with tact. There's a huge difference between "This novel's plot is stupid" and "This novel's plot is implausible."
Don't forget to include what the book did right, of course. And if you were paid in any way, from money to a free book, reveal that.