when I use writer in MS Word format I click docx to save it. when I open my document later it is in the wordpad format.
I guess you mean that when you double click on a docx document it opens in Wordpad instead of LibreOffice, right?
Just right click on the docx document, choose Open With, Choose Program, from the list select LibreOffice Writer (or Browse to the LibreOffice folder if it’s not on the list) and select the “Always use the selected program to open this kind of file” option. Click OK and it should be correctly associated.