Yes, you can say: Whatever I cook, he finds fault with it. You can also turn the sentence around and avoid using the word (preposition) " it" . He finds fault with everything I cook. He finds fault with whatever I cook. Well done! Keep up the good work 😃

Whatever I cook, he finds fault with. You don’t need the final “it”.

Yes, all three sentences are correct and natural 👍 “Whatever I cook, he finds fault with it.” - Grammatically correct. Very natural. It clearly shows repeated criticism. “You always find a flaw in everyone I introduce to you.” - Also correct. Works perfectly in a dating context. You could also say: You always find something wrong with everyone I introduce to you. (a bit more natural in everyday speech) “They would find fault with whatever I did.” - Correct. Using “would” here is great because it shows repeated behavior in the past. All three sentences make sense and sound natural. Nice work.

These sentences are all well-constructed! Finding fault with something and finding flaws in something are two ways to describe a similar behavior, and it looks like you've correctly identified that we use different prepositions with each of the two verb phrases. 😊 Do you think you could reformulate: "They would find fault with whatever I did." ... in the future?