Can I transform the follwing sentence into a yes/no question in that manner:
Sentence: the president thought that a sandwich sighed .
Yes/No question: Has the president thought that a sandwich sighed ?
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What a ridiculous sentence! Sandwiches don't sigh!!!
When we talk about "transformations" we usually mean simple grammatical manipulations that can be applied automatically.
You have made two transformations. You have taken the original
The president thought ....
Then transformed from the past tense to present perfect
The president has thought ....
And transformed that to a question:
Has the president thought ...
So you have made a yes/no question, but not the one that the teacher is expecting. Moreover the verb "think" (meaning believe) is not normally used in the present perfect. So "The president has thought..." is already odd and not idiomatic. (You can use "have thought" when it means ponder but not when it means believe)
(On the other hand the idea of sandwiches sighing is already so odd that this extra bit of oddity might be ignored)
There are lots of other transformations you could make:
Is the president thinking ...
Does the president think ...
Is the sandwich thought by the president to have sighed?
Etc. However, only one answer involves simply changing the past tense statement to a past tense question
Did the president think ...
And that is the only answer that will get you marks.