hese sentences are grammatically correct: - Would you say that London was expensive? (This is fine if you are asking someone who has visited London) - Do you think that London is expensive? (This is fine as a general question) - I would say that the weather was nice today. (This is fine when you are speaking at night) - I think that the weather is nice today. (Perfect) Could you correct me these ones? - Would you say that Manchester United lost the next match? (That just doesn't work) - Do you think that Manchester United will lose the next match? (Perfect)

Would you say + [past tense] only works when you are looking back at the past. This isn't a question of grammatical consistency. These sentences are also correct: - Would you say that London is expensive? - Do you think that London was expensive? - I would say that the weather is nice today. - I think that the weather was nice today.

Gary thank you for your message! I meant "would you say" instead of "what you say". Thanks for correcting me. What about this sentence? "Would you say that Manchester United lost the next match?" I don't know the result, and I would like to know your opinion about the result. Could you help me to undestand?

Your question is confusing, sorry. You say 'these are grammatically correct' - but they are not! Did you write 'what you say', but mean 'would you say'? Also 'would you say' usually refers to an opinion. A football result is not an opinion. It is a fact.