Python 2, 54 bytes

k='unknown'
for i in 8,6,2,0:print k[i/3:],k[i%3:]+'s'

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The results from the / and % will be [[2, 2], [2, 0], [0, 2], [0, 0]] that will be the starting indexes, removing the un when 2, keeping the string unaltered when 0

Python 2, 54 52 bytes

^{-2 bytes thanks to xnor}

k='unknowns'
for i in 8,6,2,0:print k[i/3:7],k[i%3:]

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The results from the / and % will be [[2, 2], [2, 0], [0, 2], [0, 0]] that will be the starting indexes, removing the un when 2, keeping the string unaltered when 0

Python 2, 54 bytes

k='unknown'
for i in 8,6,2,0:print k[i/3:],k[i%3:]+'s'

Try it online!

Python 2, 54 bytes

k='unknown'
for i in 8,6,2,0:print k[i/3:],k[i%3:]+'s'

Try it online!

The results from the / and % will be [[2, 2], [2, 0], [0, 2], [0, 0]] that will be the starting indexes, removing the un when 2, keeping the string unaltered when 0

Python 2, 58 bytes

k='known'
for i in 0,1,2,3:print i/2*'un'+k,i%2*'un'+k+'s'

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Python 2, 54 bytes

k='unknown'
for i in 8,6,2,0:print k[i/3:],k[i%3:]+'s'

Try it online!