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I would like respondents to be able to freely give an answer (not predetermined) and then rate it on a scale of 1-7. Unfortunately, I do not know how this should work. I would like to combine free text + rating matrix. 

First of all a little telling off: Please always answer the questions about your Limesurvey version, etc. It helps us to help you in most cases.

Now from what I understood, a respondent should be able to give a free text answer and this free text answer should then appear as a subquestion of a matrix question to rate, correct? If this is not it, please provide a little mockup survey as LSS file (not LSQ or LSG!).

So if my assumption is right, you can pipe the answer to the free text question into the subquestion text of the matrix question just like any other answer with something like this:
[code]{Q1}[/quote]

Assuming that Q1 is the free text question. You'll insert this into the place where the subquestion text should appear.