Let It Be 和 Say it ain't so/Say it isn't so

tamanmelati

Let It Be和Say it ain't so/Say it isn't so

请问上面这两句是什么意思啊？
词典给不到详尽的解释。。。
该怎样运用？

.:.阿叻.:.

据我所知
let it be-大概是指"随缘"
Say it ain't so/Say it isn't so- aint=isnt 这句话大概是指"举例说,那并非如此..."
多多指教