最贴切反映中文既然的英文是now that.
例句：He decided to go back to the front now that he had got well既然他身体已好了,他决定重新回到前线.其他如since,as也可表示既然的意思,但没有now that那么强烈.例句：Since man has done so much damage,it is up to put the matters right既然人类已经造成了这么大的损害,人类就必须纠正这种状况.As he had plenty of money he was able to help the poor既然他很有钱,他就有能力帮助穷人.