How can I restrict the answers in an open text field to only decimal numbers like 0, 0.5, 1, 1.5, etc.?
Please select the input (item) in the navigation for editing. Here select the "decimal" option for the input restriction.
How can I randomly show items of 4 questions into 1 question (Likert-scale)?
Please follow this manual: Mix Items from Different Questions
how can I only show one question at a time in the survey?
It depends what you mean by "question". If we're talking about questions labelled as such in SoSci Survey, just drag one question on each page in Compose Questionnaire.
How can I delete the database for contents ONLY IF the database already has some data ?
Do you want to delete the whole database for contents or a single entry?
In our survey, the multiple choice matrix - we get only 1/2 and not all the options. why?
A multiple choice selection or matrix requires one variable per checkbox. This is necessary to allow multiple options beein selected at the same time (and also represent that in the dataset). Therefore, you get a lot of variables, each 1/2 coded.
How can I show Page 4 only if ST01=1 and Page 5 only if ST01=2
There are multiple ways, here's a start: Filters and Conditional Questions