If a truth theory's T-sentences can be satisfied by materially equivalent but semantically distinct sentences, then knowing those T-sentences does not guarantee knowing the meaning of the mentioned sentence
Simple statements that follow the pattern 'The sentence X is true if and only if P' — a way philosophers test whether their theory of truth actually works.
Truth theory(as used in philosophy of language)
A philosophical framework that tries to explain what makes a statement true or false, and how we can know the difference.
materially equivalent(Russell's usage in his 1893 paper for Sidgwick)
mean the same as
semantics(Distinguished from metasemantics and pragmatics in Kaplan 1989)
The domain that concerns the facts about what meanings words or phrases have.
but still not know the meaning of the sentence mentioned on the left-hand side of the T-sentence. The truth of what is said by this sentence, after all, is compatible with the sentence used on the right-hand side being materially equivalent to, but different in meaning from, the sentence mentioned on the left. This seems to indicate that knowing what is said by a truth theory of the relevant kind is not, after all, sufficient for understanding a language. (For replies to these criticisms, see Da