①increase とdecrease

×Young Japanese people who go abroad are increasing.
◯The number of young Japanese people who go abroad is increasing.
増えるのは、人の【数】なので、the number of を使います。
よくある間違いで、
×The number people 【are】 increasing.
The numberが主語なので、動詞は三人称単数です♪
◯The temperature is increased.
数の要素を含んでいるものは、そのまま使ってOK.
※ちなみに、a number of にすると、沢山の人々が〜
の意味になってしまうので注意です。

②becauseの使い方

×Many young people live with their friends in apartments.Because they can share the cost and rent.
◯Many young people live with their friends in apartments because they can share the cost and rent.
※ becauseはコンマも付けずにそのまま続けるよ。

③And やButは文頭に来ない。

×Human are damaging the environment.And this has caused a lot of problems.
◯Human are damaging the environment,and this has caused a lot of problems.
×Setting Security cameras is the best way to reduce crime. But I disagree with this idea.
◯Setting Security cameras is the best way to reduce crime,but I disagree with this idea.