"worth" strikes me as an adjective after a linking verb: The treasure is worth 2 million. Consider: a) The treasure, worth 2 million, was never found. b) The treasure, having a value of 2 million,... It is hard to see worth as a verb in a) above; instead it seems to function adjectivally, like "having a value." Of course it can also function as a noun: His worth declined... How would you conjugate a verb "worth"? I worth you worth he worths Perhaps the grammar sheet writers confused 'worth" with "value": c) The treasure is valued at 2 million. "Value" is a passive voice verb in c). "Value" can be conjugated: I value you value he values --Bill Murdick