The structure and layout of the survey is shown in the Layout/Questions tab:
If you mark a question Mandatory, respondents cannot submit the survey until they have answered the question.
You can prevent respondents from returning to previous pages as they complete a survey, so that they can only move forward until they reach the end of the survey.
Note: This setting cannot be used if your survey has any mandatory questions.
Note: When previewing a survey, you are not shown any introduction message that you have set up. If you answer and save the questions you will not return any results.