While it is generally a good idea to keep text to a minimum in your survey questions, there are several scenarios where you might need to add instructions to a question. For example, you may want to add a "Mark all that apply" instruction in a checkbox question or instructions about validation rules you may have set up.
Of course, you can add this to the question text, but by using the Additional Instruction Text field instead it is easy to change the text size and formatting to call out your instructions, as well as, preserve the simplicity of your question title.
To add instructions to your question, edit your question go to the Layout tab. Enter your instruction text in the Additional Instructional Text field. By default, the text will display below the question title. If you wish to display this above the title simply check the option.
Once you are finished entering and formatting your instruction text click Save Question.