Here are the sentences with the errors identified and explained:

**65. He said me if I had done my homework.**
- **Mistake:** "said me"
- **Correction:** He told me if I had done my homework. (Use "told" instead of "said" when indicating to whom something was said.)

**66. They wanted to know how much could you pay for it.**
- **Mistake:** "could you pay"
- **Correction:** They wanted to know how much you could pay for it. (Indirect questions should follow the subject-verb order, not the question format.)

**67. You promised you will go on a picnic with us the next Sunday.**
- **Mistake:** "will go"
- **Correction:** You promised you would go on a picnic with us the next Sunday. (Past tense should be consistent with the promise made in the past.)

**68. They asked me what did happen last night, but I was unable to tell them.**
- **Mistake:** "did happen"
- **Correction:** They asked me what happened last night, but I was unable to tell them. (In indirect questions, use the statement form, not question form.)

**69. Ron said that he wasn’t sure, but the storm may stop already.**
- **Mistake:** "may stop already"
- **Correction:** Ron said that he wasn’t sure, but the storm might have stopped already. (Use "might" for past speculations and "have stopped" for past actions affecting the present.)

**70. I asked him how far was it to the station if I went there by taxi.**
- **Mistake:** "was it"
- **Correction:** I asked him how far it was to the station if I went there by taxi. (In indirect questions, the subject comes before the verb.)

**71. The shopkeeper warned the boys don’t lean their bicycles against his windows.**
- **Mistake:** "don’t lean"
- **Correction:** The shopkeeper warned the boys not to lean their bicycles against his windows. (Use "not to" for advice or warnings.)

**72. They asked me that I could do the shopping for them.**
- **Mistake:** "that I could"
- **Correction:** They asked me if I could do the shopping for them. (Use "if" for indirect questions instead of "that.")

**73. Her mother ordered her do not go out with him the night before.**
- **Mistake:** "do not go"
- **Correction:** Her mother ordered her not to go out with him the night before. (Use "not to" for orders or requests.)

**74. The traffic warden asked me why had I parked my car there.**
- **Mistake:** "had I parked"
- **Correction:** The traffic warden asked me why I had parked my car there. (In indirect questions, the subject comes before the verb.)

**75. He said that he will pick me up at 8 am the following day.**
- **Mistake:** "will pick"
- **Correction:** He said that he would pick me up at 8 am the following day. (Use "would" for future references in reported speech.)

**76. She said that the books in the library would be available tomorrow.**
- **Mistake:** "tomorrow"
- **Correction:** She said that the books in the library would be available the next day. (Reported speech should reflect a shift in time.)

**77. He advised her thinking about that example again because it needed correcting.**
- **Mistake:** "thinking"
- **Correction:** He advised her to think about that example again because it needed correcting. (Use "to" with the verb "advise.")

**78. The receptionist said I must fill out that form before I attended the interview.**
- **Mistake:** "attended"
- **Correction:** The receptionist said I must fill out that form before I attend the interview. (Present tense should be used for things that must happen in the future.)

**79. Marty said a good friendship is like a diamond – valuable, beautiful and durable.**
- **Mistake:** None. (This sentence is correct as is.)

**80. The mayor apologized on having slept in an international summit and resigned afterwards.**
- **Mistake:** "on having slept"
- **Correction:** The mayor apologized for having slept at an international summit and resigned afterwards. (Use "for" instead of "on" and "at" instead of "in.")

These corrections reflect proper grammatical structure and usage in English.