单数指的是动作（往往是抽象的）,而不是想法之类的.
the theory and practice of teaching 教学理论与实践
She's determined to put her new ideas into practice.
复数常常指“做法.做事的方式（常见的或者应该采取的方式方法）或习惯,风俗”.
如：a review of pay and working practices
.