Sure! Here are the revised sentences:

51. My brother spends a lot of time surfing the internet.
52. Ms. Lien wants to engage with young children at school.
53. His car can reach a speed of 220 km/hr, whereas my car can only reach 180 km/hr.
54. Living in the countryside is less convenient than living in the city.
55. Some forums allow teenagers to seek advice from their peers; however, my friends don’t often utilize those platforms to discuss their issues.
56. Many teenagers wish to support those in need, so they participate in community service projects.
57. Peter was not enthusiastic about doing DIY projects after school.
58. They have a keen interest in playing basketball after school.
59. I really enjoy spending time with my friends.
60. Young individuals progress more quickly than older individuals.
61. Peter is diligent; he consistently scores high on exams.
62. Mai cannot sing as well as her mother.
63. Mai reads many books; she enjoys reading.
64. We will have a brief holiday, or else we will feel quite stressed.
65. I am keen on uploading our class photos to the forum.