You can replace your formula with:

=TEXTJOIN(" ",TRUE,C1,"mastered the following suffixes:",FILTER(A2:A4,B2:B4=1))

And use the additional formula for checkboxes that aren't ticked:

=TEXTJOIN(" ",TRUE,C1,"is still working on the following suffixes:",FILTER(A2:A4,B2:B4=0))