"you act strange", instead of "you act strangely"
1. Are you all right? You act strange. Is "strange" an adjective modifying the verb "act" ?
2. He got off the bench very nervous. Is "nervous" an adjective modifyingthe verb "got off" ?
but we are taught that adjectives can not be used to modify verbs.

3. Joel Backman was trying to sleep when two guards clanged open his door. "clanged" is the verb, and what is the grammatical role for "open" here?
thank you for your reply and your time

Question

"you act strange", instead of "you act strangely"
1. Are you all right? You act strange.  Is "strange" an adjective modifying the verb "act" ?
2. He got off the bench very nervous. Is "nervous" an adjective modifyingthe verb "got off" ?
but we are taught that adjectives can not be used to modify verbs.

3. Joel Backman was trying to sleep when two guards clanged open his door. "clanged" is the verb, and what is the grammatical role for "open" here?
thank you for your reply and your time

Answer

1. This should be present progressive : 'You're acting strange'.

You could also say 'You're acting strangely' here, without changing the meaning. However, it's acceptable to use an adjective with 'act' because the meaning is the same as 'You're being strange' or 'You seem strange' or even 'You look strange' or 'You sound strange'. This is because the adjective is describing the person at that moment rather than modifying the verb.

2. He got off the bench, very nervous.

Can you see I've added a comma here? This separates the two pieces of information - 1) He got off the bench  2) (He was/looked/seemed) very nervous. The adjective 'nervous' describes the person rather than the action, and the verb ( e.g. to be) is understood. You could also say 'He got off the beach very nervously' if you want to describe the action.

3) 'Open' is an adverb.

Answer

What you're looking at is a flat adverb.

http://en.wikipedia.org/wiki/Flat_adverb