Sentence: This company is one of the most efficient in the town.

[Ecoke0005]

1) This company is one of the most efficient in the town.
==> doesn't this sentence need "a noun" after an adjective "efficient."
Or is it okay as it is since "the+adjective" works as an abstract noun, such as "the unfortunate"?

2) Generally regarded as the most influential social science treaties of the 20th century, John Maynard Keynes' book 'The General Theory of Employment Interst and Money' (had already forever changed) the way scientists looked at the economy.
==> don't understand why the past perfect tense is used here.

3) Neither proposal (remained) it its original form by the time the legislature finished its deliberations.
==> again, tense question. would the past perfect "had remained" also make sense here?

[Mister Micawber]

1) This company is one of the most efficient in the town.
==> doesn't this sentence need "a noun" after an adjective "efficient."
Or is it okay as it is since "the+adjective" works as an abstract noun, such as "the unfortunate"? This is not an abstract noun; 'efficient' is an adjective with an understood noun or pronoun: 'one of the most efficient [ones, companies] in town.

2) Generally regarded as the most influential social science treatises of the 20th century, John Maynard Keynes' book 'The General Theory of Employment Interest and Money' had already forever changed the way scientists looked at the economy.
==> don't understand why the past perfect tense is used here.-- And I don't know why there are spelling mistakes. Past perfect can only be explained by further context which you have not furnished. Presumably, the writer is predating Keynes's change to some other past event.

3) Neither proposal remained it its original form by the time the legislature finished its deliberations.
==> again, tense question. would the past perfect "had remained" also make sense here?-- Not unless there is reference to some other past event. Simple past makes it clear that change of form was completed at the same time that deliberations ended.
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Native English teacher at Mister Micawber's