Simple question, should a button be disabled if the form is not finished, i.e. still missing some input of mandatory fields? Why/Why not?
It depends on what you're working on. You can have multiple approaches here such as
- It can be disabled until completion if you're instantly validating every form field and also showing him the validation status of each field instantly.
- Show a message beside the disabled button that says "form can be submitted only upon completion" or similar.