Smash is when you break something harshly into pieces. Like, for example: "She smashed her favourite vase into pieces because she was angry." Crash is when you move into something violently and slam into it. An example is: "The man crashed his own car into a tree because he was speeding."

They're very similar, but slightly different. 'To crash' is generally used to describe the thing moving at a high speed, while 'to smash' describes the person or thing doing the breaking. For example, if I smash a glass with my fist, then my fist crashed into the glass. 'Crash' sounds a little unnatural in this example, just because 'to crash' is usually only used to describe vehicles or things moving at a very high speed. It's possible for something to crash into something else without smashing it. Something only smashes if it breaks violently into smaller pieces, but 'crash' only implies that the moving object slammed into something very hard.

smash can be when you violently break something into pieces or when you push down on something. "He stomped on the phone, smashing it to pieces." or "She smashed the avocado in order to make guacamole." crash is when two objects collide. "She threw the plate, causing it to crash into the wall."