Concord as a scope of English Language, can be referred to as "agreement". The types of concord include : Subject/verb concord (grammatical concord), proximity concord, notional cord, pronoun-antecedent concord, verb-tense concord, "inter alia".
However, I'd like to elucidate on the pre-dominant rules of the Subject/verb concord only. This adduces to the fact that it is the most employed in our written and verbal English.

What is Subject/Verb Concord?
This suggests that there must be an agreement between the subject and the verb in terms of numbers and persons. The core rule of the Subject/verb concord states : Singular subject + Singular Verb; plural subject + plural verb.
1a) The governor accompanied by his wife "has" arrived. (In this context , the governor (singular) is the main subject of the sentence)
b)The "proprietor and principal" has a new car( the proprietor is likewise regarded as the principal. Hence, we have a singular subject)
c)The proprietor and the principal have a new car (in this case, the proprietor is "not" the principal. Hence, we have a plural subject)
d) Rice and beans is good
e) The "causes" of the crisis "are" unknown
F) The "cause" of the crisis "is" unknown.

2) When the subject is a noun clause, the rule takes 3rd person singular agreement
a) "Who the man was" remains a mystery
b) "How the man solved the problem" is still unknown

3) when a sentence begins with "there" or "here" , the verb is determined by the object.
a) There goes Peter(singular object)
b) There "go" Peter and Paul (plural object)

4) When the determiner "Many a/an" is used to introduce a sentence, the noun and verb should be singular e.g. "Many a student" is unserious

5) Invariable singular nouns (e.g civics, physics, economics, darts, measles, politics, gymnastics, shingles, athletics) that are used as a subject should take a singular verb in agreement.
a) Gymnastics "is" an exciting sporting activity
b) politics "has" its good side.

6) Summation Plural (trousers, pants, scissors, glasses, tongs, pincers, pliers, binoculars) when used as a subject of the sentence should take plural verb form in agreement.
a) The scissors "are" sharp
B) Your trousers "are" dirty
Should it be a "of" or "partitive" phrase used alongside the summation plural noun, it takes either singular or plural verb
a) That pair of scissors is sharp(singular verb "is")
b) Those pair of scissors are sharp(plural verb "are")

7) When invariable plural nouns(surroundings, contents,funds,remains,whereabouts, damages,odds, premises) are used as subject , they normally take plural verb form e.g.
a) The odds "are" against him
b) His surroundings "are" messy
c) His whereabouts "are" unknown.

When the subject of a sentence is a noun phrase which precedes a prepositional phrase, the verb form should be determined by the noun(phrase).
a)" The fight" against bribery and corruption "is" a must win.
b) "The causes" of the crisis "are" unknown

9)Indefinite pronouns like someone, something, everyone, nobody, take singular verb in agreement. E.g. a) Nobody is leaving this premises
b)Everybody has seen the President

10)Indefinite pronouns such as few, a few, many, several, some, both, all take plural verb in agreement
A) A few children were seen loitering the vicinity.

11) If the subject is introduced by "a number of" , it takes a plural count noun and plural verb form.
A) A number of "students"(plural)have(plural verb) registered for the session

12) "More than" + Singular count noun + singular verb
a) More than one bag of rice "is" enough

"More than" + Plural count noun + plural verb
b) More than ten bags or rice "are" enough.

"More than" + non-count + Singular verb form
C) "More than" water "is" required to extinguish the fire.

ADDITIONAL:
PROXIMITY CONCORD
When two nouns(or pronouns) are combined with the conjunction "either.....or" , "neither....or", "or", the noun nearest to the verb determines the verb used
A) Neither the teacher nor his pupils(plural) "are" guilty
B) Either the student or their teacher(singular) "is" our guest.
C) You or I(singular) "am" invited to the party.

In addition to the above, you can also find simple explanations on concord in English, with practice exercise at:

https://www.nairaland.com/5663529/english-concord-rules

Great

Nice job

Nice