"decide to do something" 表示做出决定，决定去做某事。例句：I decided to take a break from work and go on vacation.（我决定休息一下，去度假。）

"expect to do something" 表示期待或预计要做某事。例句：I expect to finish my project by the end of the week.（我期望在这周末之前完成我的项目。）

两者的区别在于，"decide to do something" 强调的是做出了决定，而"expect to do something" 强调的是一个预期或期待。