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)