I am confused in below.Could anyone please help me which one is correct and why ? "finally,blinds are fitted" "Finally,blinds has been fitted." "Finally,blinds have been fitted."
This is basic stuff: These phrases are all in 'passive' (not 'active') voice. The verb (fitted) is 'perfect' (completed), and is preceded by auxiliary verbs that handle the tense, aspect and subject agreement. Since the subject (blinds) is plural, the first [aux] verb should not have the 's' suffix. (The 's' on the verb is a type of parity with the subject.)
There is also a type of parity which signals 'passive': the total of 'perfect' and 'possessive' verbals is odd ('are fitted' = 1 perfect; 'have been fitted' = 1 possessive + 2 perfect; 'are being fitted' = 1 perfect; 'have been being fitted' = 1 possessive + 2 perfect). Active phrases have even parity: ('fitted' = 0 perfect = 0 aux + 1 past; 'have fitted' = 1 possessive + 1 perfect; 'have been fitting' = 1 possessive + 1 perfect). Note that the very first [aux] verbal is never 'perfect', but it may be 'past' (was, had, etc).
These parities are not something that we are aware of when we talk.
"Blinds are fitted" is good. The verb "are" is the present plural form of the verb "to be." "Fitted" is a predicate adjective, the past participle of the verb "to fit" used as an adjective. It expresses a present condition.
"Blinds have been fitted" is also good. The verb "have been fitted" is the past perfect progressive form of the verb "to fit." It expresses a past event that continues to the present.
The plural subject "blinds" must take a plural verb "have." "Blinds has" is wrong.