"I won't notice" vs "I wouldn't have noticed"

Question

https://data.grammarbook.com/blog/verbs/if-i-would-have-vs-if-i-had/

After searching on Google and reading examples of subjunctives on the link above, I'm still not sure if I should use subjunctive in this manner.

Context: I asked a question about Problem A. Someone helped me and told me there's another problem with B. Then I would like to reply and thank him for telling me what I didn't notice before.

Is it appropriate to use the subjunctive or not for the situation?

Thank you. I won't notice the problem B if you don't mention that.

Thank you. I wouldn't have noticed the problem B if you hadn't mentioned that.

Answer 1

You must use the subjunctive because the situation you are supposing is contrary to the fact. 'You' did mention the problem B. It is just that you are supposing the situation that is opposite to that and making an assumption of the event in the hypothetical, contrary-to-fact situation. You must use the form. (simple past 'mentioned that' to past perfect 'had mentioned that', and 'didn't notice' to 'wouldn't have noticed') The modal verb in its past form 'would' (from will) indicates events in a situation that is contrary to the fact, and it's the same with other modal verbs in their past forms(can/shall/may to could/should/might).