When working a KNIT STITCH, make sure the working yarn is dangling in the back of the work.
When working a PURL STITCH, make sure the working yarn is dangling in the front of the work.
If you don't, in either case, you will be making/creating an extra stitch. Sometimes this is done 'on purpose'...and in those cases, this maneuver is called a yarn over (abbreviation "yo
Please click on this link
...and scroll down to the abbreviation yo
in our Knitting Help GLOSSARY...and watch Amy's video for doing a yarn over.
You will see for yourself what happens when you make a knit stitch with the yarn in the front of the work instead of the back of the work, and vice versa.
Hope this helps!