To add a sub-question, you have two options: create a new sub-question or turn an existing question into a sub-question.
Create a sub-question
To add a sub-question, please click on the green icon "Add sub-question" and then select the item you need.
Take a look at our item list if needed: Items description.
Fro better clarity, let's take an example: How to add an "Other" field as a sub-question?
Your primary question is a "Checkbox" question type, and one of the answers is "Other". This answer is here to let your respondent tell you more about what he thinks.
Click on the green icon "Add sub-question" and select "Text field". Set it to: "When / equal to / Other".
Modify the title of your question with something like "Please specify" and Validate.
You can see here that when "Other" is selected, the sub-question "Please precise" appears.
Turn an existing question into a subquestion
In your survey, pick the question you want to turn in a sub-question, then on the question number, click on the right arrow:
Here the "question 2" becomes "question 1.1". That means the question 1.1 is now dependent (is a child) of the question 1.
To go back (turn back a sub-question in a question), click on the left arrow to extract the question 1.1 of its parent question (question 1).
In our example we get the initial statement of the question 2.
Don't forget to save your survey when you're done.