Tiempos para stiffen Present participle:
stiffening
Present:
stiffen (3.person: stiffens)
Past:
stiffened
Future:
will stiffen
Present conditional:
would stiffen
Present Perfect:
have stiffened (3.person: has stiffened)
Past Perfect:
had stiffened
Future Perfect:
will have stiffened
Past conditional:
would have stiffened