紅筆
which 後面個動詞要唔要加 s？
✗ 你會咁寫
They can go cycling which is not only reduce screen time but also increase exercise.
✓ 應該係
They can go cycling, which not only reduces screen time but also increases exercise.
「which is not only reduce」係錯嘅：which 後面直接用動詞就得，唔使加 is。因為指緊嘅係單數，所以 reduce 同 increase 都要加 s。
紅筆
點解你會咁寫
中文冇關係子句，所以呢個結構冇得靠語感。寫完 which 之後停一停，問自己：呢個 which 指緊上面邊個字？跟住個動詞就跟返嗰個字。
紅筆
再睇多兩個例子
✗ I have a friend who live in Japan.
✓ I have a friend who lives in Japan.
✗ Students which is lazy will suffer.
✓ Students who are lazy will suffer.
紅筆