1. Xiaoming said, "If I don't know, Xiao Qiang certainly doesn't know."

The subtext of this sentence is actually: "I should guess right. If I guess wrong, Xiao Qiang will definitely not know." But Xiao Ming is still not sure whether he guessed correctly. He needs Xiao Qiang to prove it. What value does m take for Xiao Ming to say so? Obviously, neither 6 nor 12 is desirable. If m is 6 or 12, n may be 2 or 7-Xiao Qiang can know Mr. Zhang's birthday by the number 2 or 7. Then m can only be 3 or 9, and n can only take the values in 1, 4, 5 and 8.

If m is 3, n can take three values, and the result becomes "If Xiao Ming doesn't know, Xiao Qiang may or may not know (2-4, 3-8)." In this case, Xiao Ming's statement that "if I don't know, Xiao Qiang certainly doesn't know" is not valid, and Xiao Ming is not confident enough to say so.

If m is 9, Xiao Ming knows that n can only be 1 or 5. At this point, Xiao Ming's guess is N= 1, and Xiao Ming is not sure whether n is 1. If n is not 1 but 5, Xiao Ming said, "If I don't know, Xiao Qiang certainly doesn't know." At this point, in fact, Xiao Ming already knows that there are only two situations, only waiting for Xiao Qiang to confirm whether n is 5.

Xiao Qiang said, "I didn't know at first, but now I know."

Xiao Qiang said, "I didn't know", and verified that N is really neither 2 nor 7. At the same time, Xiao Qiang also knew that "m is not 6 or 12, only 3 and 9 are left in m". If n is 5, Xiao Qiang should say, "I didn't know at first, but I still don't know." According to the inference in the first section, N= 1, so Xiao Qiang can say, "I didn't know it at first, but now I know it."

Xiao Ming said, "Then I know."

Xiao Ming is waiting for Xiao Qiang. No matter how Xiao Qiang answers, Xiao Ming will know the correct answer. If Xiao Qiang says "I don't know yet", then Xiao Mingcan still knows that "only N=5 will make Xiao Qiang feel at a loss", so the answer is September 5; If Xiao Qiang said "I know", it must be 1 September.

In fact, from beginning to end, Xiao Ming knew that all he needed was Xiao Qiang to verify his guess. For Xiao Ming, it is a multiple-choice question that is not A or B. Therefore, according to the story development clue of the topic itself, Xiao Ming's third sentence is unnecessary, but many people will use this condition when deriving-it is a bit like doing a math problem.