Module English: Singular and plural nouns

The verb must agree with its subject in number and person. That is the verb must be of the same number and person as its subject. If the subject is plural, the verb must be in the plural. Similarly, if the subject is singular the verb must be singular.
Incorrect: Petrol or kerosene are used.
Correct: Petrol or kerosene is used.
Two singular nouns joined by or take a singular verb.
Incorrect: A melon or grapes is suitable.
Correct: A melon or grapes are suitable.
Correct: Grapes or a melon is suitable.
When a singular noun and a plural noun are joined by or, the verb agrees with the nearest noun – it would be better to use the plural noun second; then a plural verb would be used.
Incorrect: One of my students sing well.
Correct: One of my students sings well.
Incorrect: One of the eggs are bad.
Correct: One of the eggs is bad.
A singular noun should be used after one of and a plural noun.
Incorrect: Bullets or a bomb are not allowed on the plane.
Correct: Bullets or a bomb is not allowed on the plane.
Correct: A bomb or bullets are not allowed on the plane.
Incorrect: Hundred dollars are too much to lose.
Correct: Hundred dollars is too much to lose.
When the subject is a sum of money considered as a whole, the singular verb is used.
Source: http://www.englishpractice.com/improve/singular-plural-nouns/

Module English: Uses of the relative pronouns

Uses of the relative pronouns

As a general rule, who is used for persons only. It may refer to a singular noun or a plural noun.
The student who works hard will win. (Here who refers to the singular noun student.)
The students who work hard will win. (Here who refers to the plural noun students.)
He who hesitates never wins.
They never fail who die for a cause.
Note that who is sometimes used to refer to animals.
Whose
Whose is the possessive form of who. It is used in speaking of persons, animals and inanimate objects.
This is the question whose solution has baffled thinkers of all ages. (More common: This is the question the solution of which has baffled thinkers of all ages.)
The girl in whose pool we swam was very sweet.
Which is used in speaking of animals and objects without life. It may refer to a singular noun or a plural noun.
An opportunity, which is lost, is lost for ever.
This river, which flows through London, is called Thames.
The hotel, at which we stayed, was the cheapest in the town.
Which may also refer to a clause.
The man was said to be mad, which was not the case.
He is not here, which is unfortunate.
He said he hadn’t taken the money, which was a lie.
Source: http://www.englishpractice.com/grammar/relative-pronouns-4/