Here are the corrections for the mistakes in the sentences:

33. Walking in the rain **is** not a good idea.
- Correction: "are" should be "is".

34. The unemployed really **need** to be given more help.
- Correction: "needs" should be "need".

35. To learn foreign languages **is** necessary for us to have a good job.
- Correction: "are" should be "is".

36. The police **are** asking him a lot of questions which he can't answer.
- Correction: "is" should be "are".

37. Life is not easy for those who **are** without jobs.
- Correction: "is" should be "are".

38. The Chinese **are** very famous for their food.
- Correction: "is" should be "are".

39. I love swimming which **is** a good sport for my health.
- Correction: "are" should be "is".

40. All the books on the top shelf **belong** to me.
- Correction: "belongs" should be "belong".

41. One of the biggest problems facing our society **is** unemployment.
- Correction: "are" should be "is".

42. A few of the audience **are** enjoying every minute of the performance.
- Correction: "is" should be "are".

43. How you got there **does** not concern me.
- Correction: "do" should be "does".

44. What I say about these problems **is** my own affair.
- Correction: "are" should be "is".

45. A great deal of money **has** been spent on the books.
- Correction: "have" should be "has".

46. Either you or the headmaster **is** handing the prizes to these gifted students at the meeting.
- Correction: "are" should be "is".

47. The poor woman with her two children **was** seen begging in a street corner.
- Correction: "were" should be "was".

48. Happiness and success **depend** on your ability and luck.
- Correction: "depends" should be "depend".

49. There are forty students in the class. Half of the class **are** boys.
- Correction: "is" should be "are".

50. The Vietnamese **are** hard-working, intelligent and brave.
- Correction: "is" should be "are".