Those who were able took advantage of the opportunities for

我仔细看了，从另一个角度来分析，句子没有问题： Those （who were able ）took advantage of the opportunities for success and happiness (that presented themselves). 本句有两个（括号里的）限制性定语从句，修饰Those (people), 和 the opportunities for success and happiness....暂时别管它了。 把定语从句部分抽掉，留下主句（主干）部分： Those...took advantage of...那些善于把握……的人 全句的大意是：有才华的人总是在成功和幸福的机遇到来时及时地抓住（这些良机）。