貧しかったので

because [someone] was poor

→ 貧しい
phrase Rare #42,144
because [someone] was poor 貧しかった is the past form of 貧しい (poor), followed by ので (because).
JA: 彼女は貧しかったので、大学に行けませんでした。
EN: Because she was poor, she couldn't go to college.

彼らは貧しかったので懸命に働かなければならなかった

Because they were poor, they had to work hard.

彼は貧しかったので大学へ行けなかった

He was poor, so he couldn't go to college.

彼はとても貧しかったのでそのパンを買えなかった

He was so poor that he couldn't afford to buy the bread.