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Take notes on what the professor said in your French class.
Literal
French-language [gen-の] class [at-で] professor [subj-の] said thing [about-について] notes [obj-を] take [command-なさい].
A neat example of の-for-が substitution inside a relative clause: 教授の言った 'what the professor said' — の replaces the expected が as the subject marker. This substitution is productive only in relative clauses and some noun-modifying contexts. について continues to mark 'regarding.' ノートを取る is the set verb-noun collocation for 'to take notes.'