“可能性极大”可以翻译为以下几种英语表达，具体使用哪种可根据语境和表达习惯选择：
1、 There is a high/strong probability：
示例：There is a high probability that it will happen.（这件事发生的可能性极大。）
2、 It's highly/very likely：
示例：It's highly likely that he will succeed.（他成功的可能性极大。）
也可以说成：It's very likely that something will occur.（某事发生的可能性极大。）
3、 The chances are extremely good：
示例：The chances are extremely good that we'll win the game.（我们赢得比赛的可能性极大。）
4、 There's a great/strong chance：
示例：There's a great chance that he'll accept the offer.（他接受这个提议的可能性极大。）
5、 It's almost certain：
示例：It's almost certain that he'll come.（他来的可能性极大，几乎可以肯定。）