Related Clues:

• Sometimes they're perfect
• Tautens; verb forms
• Future, present and past
• Past, present etc.
• Reacts to a strange noise, perhaps
• The strains of the past - and present and future
• Tightens up when seeing perfect future in front of them!
• Future or past, say
• Just one of them is perfect
• They can only be perfect in the past :(
• Stiffens
• Conjugation factors
• Language class lessons
• Conjugating categories
• They can be imperfect