Here are the corrections for each sentence along with the explanation of the mistakes:

**33. Original sentence:** He said that they will have to set off soon after midnight.
**Correction:** He said that they would have to set off soon after midnight.
**Explanation:** In reported speech, the future tense "will" should be changed to "would" to maintain the correct sequence of tenses.

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**34. Original sentence:** When he lived in Japan, he didn’t used to eat food with forks, but with chopsticks.
**Correction:** When he lived in Japan, he didn’t use to eat food with forks, but with chopsticks.
**Explanation:** The correct form is "didn't use" instead of "didn’t used." The auxiliary verb "did" requires the base form "use."

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**35. Original sentence:** He thinks that geometry is not as difficult than arithmetic.
**Correction:** He thinks that geometry is not as difficult as arithmetic.
**Explanation:** The correct comparative structure is "as...as," not "as...than." The phrase should compare entities directly using "as" for both sides of the comparison.