1. Certain nouns being a singular form represent plurality and therefore, take a plural verb in a sentence.
Example: police, clergy, people, peasantry, cattle.
People has left. (Incorrect)
People have left. (Correct)
2. Certain nouns take the plural verb because of their plural form.
Example: Clothes, Scissors, trousers, amends, spectacles, thanks.
The spectacles is missing. (Incorrect)
The spectacles are missing. (Correct)
3. When a noun denoting weight, number, money, length or measure is following a number, the noun form does not change as long as another noun or pronoun follows it.
Example:  Million, pair, meter, year, dozen.
This is a 9-meters scale. (Incorrect)
This is a 9-meter scale. (Correct)
4. When the pronoun ‘one’ is used, it should be maintained throughout the whole sentence.
One must help his siblings.  (Incorrect)
One must help one’s siblings. (Correct)