How is my mum going to think if she knew that I bunked off school to study English, IGCSE and A-level.
Is the rest of the sentence natural?
How What is my mum going to think if she finds out knew that I bunked off school to study English, IGCSE and A-level. is . .
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How What is my mum going to think if she finds out knew that I bunked off school to study English, IGCSE and A-level.
is . . . going to think is future and knew is past, so you need to change that.
You don't need for.