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Can anybody explain(using the examples below) why some teachers say that if "all or what" is subject of a sentence, it can take a plural verb as well as a singular verb?

1. All I did at the party was watch TV alone.

(Why 'was'? Because of the verb in the complement?)

What you should do now is quit smoking for your health.

(Why 'is'? Because of the verb in the complement?)

2. All we found in the storage room were old books.

All he needs at the moment is courage and confidence.

(Then why 'is'? Is 'are' also correct? The complement consists of two nouns!)

What they desperately want is clothes and shelter.

(Then why 'is'? Is 'are' also correct? The complement consists of two nouns!)
Comments  
Your linking verb should agree with the subject. The subjects in your sentences seem to be the ones after the linking verbs. This rule may not be followed in certain cases. (e.g. if after the linking verb the subject is pronominal such as YOU and US).
Joon22571. All I did at the party was watch TV alone.
(Why 'was'? Because of the verb in the complement?)
What you should do now is quit smoking for your health.
(Why 'is'? Because of the verb in the complement?)
The verbs in base infinitive form are actually your subjects. These are just cases of 'inverted sentence'.

(To) watch TV was all I did at the party.
(To) quit smoking is what you should do.
Joon22572. All we found in the storage room were old books.
(Why 'were'? Because of the plural noun in the complement?)
What children need today are books rather than toys.
(Why 'are'? Because of the plural noun in the complement?)
Your subjects (old books; books) are in plural form. Thus a plural verb
Joon2257 What they desperately want is clothes and shelter.
(Then why 'is'? Is 'are' also correct? The complement consists of two nouns!)
This should be ARE.