1. That's why it's expensive.
だから高いんだ

2. That's why it's better to take action.
だから行動に移した方が良いのだ

3. That's why we think we have to be more careful.
だから私達はもっと慎重になった方が良い

4. That's why we're bringing our umbrellas.
だから私達は傘を持ってきたのです

5. That's why it's time to start studying.
だから勉強を始める時間なのです