Here are the corrections for the mistakes in the sentences:

1. **Neither** his parents nor his teacher **was** satisfied with his result when he was at high school.
*(Correction: "were" → "was")*

2. You should eat **fewer** sweets because you may have a toothache.
*(No correction needed; this sentence is correct.)*

3. Either the doctor or the nurses **take** care of changing the patients' bandages.
*(Correction: "takes" → "take")*

4. Every student who **majors** in English **is** ready to participate in the oratorical contest.
*(Correction: "are" → "is")*

5. Don’t drink too **much** milk and you will gain weight.
*(Correction: "your weight" → "weight")*

6. All the books on the top shelf **belong** to me.
*(Correction: "belongs" → "belong")*

7. Happiness and success **depend** on yourself.
*(No correction needed; this sentence is correct.)*

Final corrected version:

1. Neither his parents nor his teacher **was** satisfied with his result when he was at high school.
2. You should eat fewer sweets because you may have a toothache.
3. Either the doctor or the nurses **take** care of changing the patients' bandages.
4. Every student who **majors** in English **is** ready to participate in the oratorical contest.
5. Don’t drink too **much** milk and you will gain weight.
6. All the books on the top shelf **belong** to me.
7. Happiness and success **depend** on yourself.