Firstly from to "start from scratch" is an idiom meaning an expression. In this case the Idiom is used to express a restart or could also mean to begin something from nothing. He started his speech from scratch with no assistance. I will have to start from scratch because the design has too many flaws. While start from the beginning won't indicate a redo or repeat it would basically mean beginning from the start of something that already exists. The teacher asked us to read from the beginning. Basically the starting point...

'From scratch' is an idiom and is a bit more informal than 'from the beginning'. 'From scratch' suggests that you have either restarted something (possibly due to failure the first time) or that you have made something with very few materials.

FROM SCRATCH comes from cooking. For example, if you make a cake from scratch that means you used flour, sugar, eggs, etc, NOT a cake mix from a box.

Start from scratch=重新开始 start from the beginning =从头开始、不一定表示一个重做或重复，比如看电影的时候 you start from the beginning