Word: 既然 (jìrán)

你既然已经知道了，还问什么？

Nǐ jìrán yǐjīng zhīdàole, hái wèn shénme?

If you already know, why are you still asking?

既然你已经开始学钢琴了，那还是把它学会吧。

Jìrán nǐ yǐjīng kāishǐ xué gāngqín le, nà háishi bǎ tā xuéhuì ba.

Since you've already started learning to play the piano, then you should \[see it through and\] learn it well.

既然没有牛奶，咱们就喝黑咖啡算了。

Jìrán méiyǒu niúnǎi, zánmen jiù hē hēikāfēi suànle.

Since there's no milk, let's just drink black coffee.