Rule 7. Use a singular verb with distances, periods of time, sums of money, etc, when considered as a unit.
Examples:
Three miles is too far to walk.
Five years is the maximum sentence for that offense.
Ten dollars is a high price to pay.
BUT
Ten dollars (i.e., dollar bills) were scattered on the floor.

Rule 8. With words that indicate portions—eg, a lot, a majority, some, all—Rule 1 given earlier in this section is reversed, and we are guided by the noun after of. If the noun after of is singular, use a singular verb. If it is plural, use a plural verb.
Examples:
A lot of the pie has disappeared.
A lot of the pies have disappeared.
A third of the city is unemployed.
A third of the people are unemployed.
All of the pie is gone.
All of the pies are gone.
Some of the pie is missing.
Some of the pies are missing.
NOTE
In recent years, the SAT testing service has considered none to be strictly singular. However, according to Merriam-Webster’s Dictionary of English Usage: “Clearly none has been both singular and plural since Old English and still is. The notion that it is singular only is a myth of unknown origin that appears to have arisen in the 19th century. If in context it seems like a singular to you, use a singular verb; if it seems like a plural, use a plural verb. Both are acceptable beyond serious criticism.” When none is clearly intended to mean “not one,” it is followed by a singular verb.

Rule 9. With collective nouns such as group, jury, family, audience, population, the verb might be singular or plural, depending on the writer’s intent.
Examples:
All of my family has arrived OR have arrived.
Most of the jury is here OR are here.
A third of the population was not in favor OR were not in favor of the bill.
NOTE
Anyone who uses a plural verb with a collective noun must take care to be accurate—and also consistent. It must not be done carelessly. The following is the sort of flawed sentence one sees and hears a lot these days:
The staff is deciding how they want to vote.
Careful speakers and writers would avoid assigning the singular is and the plural they to staff in the same sentence.
Consistent: The staff are deciding how they want to vote.
Rewriting such sentences is recommended whenever possible. The preceding sentence would read even better as:
The staff members are deciding how they want to vote.

Rule 10. The word were replaces was in sentences that express a wish or are contrary to fact:
Example: If Joe were here, you’d be sorry.
Shouldn’t Joe be followed by was, not were, given that Joe is singular? But Joe isn’t actually here, so we say were, not was. The sentence demonstrates the subjunctive mood, which is used to express things that are hypothetical, wishful, imaginary, or factually contradictory. The subjunctive mood pairs singular subjects with what we usually think of as plural verbs.
Examples:
I wish it were Friday.
She requested that he raise his hand.
In the first example, a wishful statement, not a fact, is being expressed; therefore, were, which we usually think of as a plural verb, is used with the singular subject I.
Normally, he raise would sound terrible to us. However, in the second example, where a request is being expressed, the subjunctive mood is correct.

Note: The subjunctive mood is losing ground in spoken English but should still be used in formal speech and writing.
The end
Source: grammarbook.com