This is not "từ", this is a saying. "Lên non mới biết non cao": non = núi = a mountain. This sentence literally means that you dont know how high a mountain is until you climb it. "Có con mới biết công lao mẹ già". This one means literally that you dont know the hardship of your mother until you have a baby. So this saying implies that you dont really know how difficult something is until you do it, deal with it. Hope you understand my explanation ^^