Some sentences might be right, but just aren’t, and it seems a bit weak to say to students “It just isn’t”. One sentence in a grammar review was “I ____ pass the exam”. Alongside one obviously wrong choice were “think she won’t” and “don’t think she’ll”. Several students chose “I think she won’t pass”.
Actually, it’s not wrong. It’s perfectly grammatical, and makes more sense – I think + she won’t pass the exam compared with I don’t think + she will pass the exam (clearly, I do think something) – and could be used for emphasis: I think she won’t pass the exam. But it is used way less that I don’t think she’ll (and equivalent sentences with all the other pronouns (per Google Ngrams)), which is the usual/natural choice. Saying I don’t think she’ll pass doesn’t mean I think she won’t pass, but that I think she might or mightn’t pass. I’ve said equivalent sentences to my wife, who has picked up on the second half of the sentence without processing the “I don’t think”. PS Australia has had a very long and late summer, but some nights recently have been cooler. As I was drafting this post, my wife asked “Will we need an extra blanket?”. I replied “I don’t think so”.
A similar question required students to put the given words into order. The expected, usual/natural sentence was “Who do you think is going to win the next election?”. One student wrote “Do you think who will win the next election?”. This is wrong – the question word must come first, but compare “Do you think [name/party] will win the next election?”.