Brain Teasers and Puzzles

A box for your workings out:

Hint: Some are much more common than others.

Answer:
Continuum, continuums, residuum, residuums, vacuum, vacuums, vacuumed, vacuuming, muumuu, muumuus.

There are a few more less common words such as menstruum, menstruums, triduum, triduums, weltanschauung, weltanschauungen.