Thank you for replying me.
The thing I am curious about is in sentence A, how the plural form of the subject and verb(kids, are) takes the singular form of the noun as a complement (a full time job).
I thought that was not grammatical like sentence B.
A: Kids are a full time job.
B: Apples are a tasty fruit.
***** NOT A TEACHER *****
Good morning, infiniteone.
(1) Thank you for asking such a difficult question. I think I may have found some helpful information.
(2) Yes, you are correct about the rule:
singular noun (or phrase) + IS + singular noun.
Plural noun + ARE + plural noun.
(3) As you know, probably every language has EXCEPTIONS.
(4) You wish to talk about a plural noun (apples) and a singular noun (fruit).
(a) The "rule" is usually this: the verb agrees with the FIRST noun:
(i) Apples + ARE (because "apples" is plural) + a tasty fruit.
(ii) A tasty fruit + IS (because "fruit" is singular) + apples.
(P. S. You may also say: The apple is a tasty fruit; A tasty fruit is the apple.)
(5) Your sentence "Kids are a full-time job" is a little more difficult.
(a) "Are" because "kids" is plural.
(b) But probably you may NOT say: "A full-time job is kids."
(c) In your "apples" sentence, you were identifying something. So you could say "Apples are...." or "A tasty fruit is..."
(d) If I understand my book correctly, your "kids" sentence is not identifying anything. It is describing something -- as an ADJECTIVE does.
(e) When you say "full-time job," I think that you mean it takes all of your time.
(i) Well,then, your sentence almost equals "Kids are time-consuming." As you can see, "time-consuming" is an adjective.
(a) The book I am using has two such examples:
The younger children are a problem.
Dogs are good company.
"Are a problem" and "good company" LOOK like nouns, but the MEANING is clearly ADJECTIVAL. (And, as you know, an IS/ARE sentence with adjectives does not have to be "balanced": He IS nice; They ARE nice.)
Thank you for your great question. I learned so much.
If you want to read about this yourself, see A COMPREHENSIVE GRAMMAR OF THE ENGLISH LANGUAGE, by Randolph Quirk , Sidney Greenbaum, Geoffrey Leech, and Jan Svartvik. I have the 1985 edition. There may be a newer edition. Look for "subject-verb concord."