He speaks very clearly to make himself understood.
He speaks very clearly to make his speech understood.
He speaks very clearly to make his speech easy to understand.
He speaks very clearly to make his speech easily understood.
麻烦老师帮忙看下，上述表达是不是都正确，尤其是第三句为何要加to？
从语法上看，四句都是对的。
第三句之所以加 to，是由于在 easy 后面要接不定式。
