"Finally, the students tested on the word pairs they studied. The students mind wandered more while studying more difficult and easy word pairs, compared with medium difficulty."
Firstly, we need to clarify whether it is one or more students whose mind(s) wandered. That will affect plurals and apostrophes! I will assume it is intended for there to be more than one.
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Firstly, we need to clarify whether it is one or more students whose mind(s) wandered. That will affect plurals and apostrophes! I will assume it is intended for there to be more than one.
I think this sounds natural and keeps your format almost completely:
Finally, the students WERE tested on the word pairs they HAD studied. The students' minds wandered more while studying more