The simple sentence 7.18

Concord
7.18
Subject-verb concord
The most important type of concord in english is concord of number between subject and verb. Thus (3) and (4) are ungrammatical :
(1) the window is open (sing+sing)
(2) the windows are open (plur+plur)
(3) *the window are open (sing+plur)
(4) *the windows is open (plur+sing)
A clause in the position of subject counts as singular for purposes of concord : how they got there doesn't concern me, to treat them as hostages is criminal. The same is true of prepositional phrases, etc, acting as subject. After the exams is the time to relax, etc. Nominal relative clauses on the other hand, since they are equivalent to noun phrases (11.13), may have plural as well as singular concord : what were once human dwellings are now nothing but piles of rubble.
Note :
(A) in fact, it is possible to generalize the rule as 'A subject which is not definitely marked for plural requires a singular verb'. This would explain, in addition to clausal and adverbial subjects, the tendency in informal speech for is / was to follow the pseudo-subject there in existential sentences sich as there's hundreads of people on the waiting list (14.19 ff).
(B) apparent exceptions to the concord rule arise with singular nouns ending with the -s of the plural inflection (measles, billiards, mathematics, etc, 4 32), or conversely plural nouns lackingvthe inflection (cattle, people, clergy, etc, 4.35) :
Measles is sometimes serious        our people are complaining
(C) plural words and phrases (including coordinate phrases, sec 7.21) count as singular if they are used as names, titles, quotations, etc (see further 9.57) :
Crime and punishment is perhaps the best-constructed of Dostoyevsky's novels ; but the brothers karamazov is undoubtedly his masterpiece. 

' the cedars' has a huge garden

' senior citizens' mean, in common parlance, people over sixty

the titles of some works which are collection of stories, etc, however, can be singular or plural : the canterbury tales exist / exists in many manuscripts.

7.19

notional concord, and proximity

two factors interfere with concord as presented in 7.18. 'notional concord ' is agreement of verb with subject according to the idea of number rather than the actual presence of the grammatical marker for that idea. thus the government is treated as plural in the government have broken all their promises (BrE), as is shown not only by the plural verb have, but also by the pronoun their.

7.21 coordinated subject

the principle of 'proximity' denotes agreement of the verb with whatever noun or pronoun closely precedes it, sometimes in preference to agreement with the headword of the subject :

no one except his own supporters agree with him

one in ten take drugs

7.20

collective nouns

in BrE, collective nouns, notionally plural but grammatically singular, obey notional concord in examples such as the following where AmE usually has the singular :

the public are tired of demonstrations                                                  (1)

the audience were enjoying every minute of it                                       (2)

although sigular and plural verbs are more or less interchangeable in these contexts, the choice is based, if on anything, on whether the group is being considered as a single undivided body, or as a collection of individuals (cf 4.62). thus plural is more likely than singular in (2), because consideration is being given to the individual reactions of members of the audience. contrastingly, singular has to be used in sentences like the audience was enormous.

coordinated subject

7.21

when a subject consists of two or more noun phrases coordinated by and, a distinction has to be made between appositional and non-appositional coordination. under NON-APPOSITIONAL COORDINATION we include cases that can be treated as an implied reduction of two clauses. these have a verb in the plural :

tom and jerry are now ready ( <---> tom is now ready and mary is now ready)

what i say and what i think are my own affair ( <---> what i say is .... and what i think is ... )

a singular verb is used with conjoinings which represent a single entity : 

the hammer and sickle was flying from a tall flag pole

conjoinings expressing a mutual relationship, even though they can only indirectly be treated as reductions of clauses in this way, also take a plural verb:

your problem and mine are similar ( <---> your problem is similar to mine and mine is similar to yours)