Opera is actually a plural; the singular is opus, meaning "work." But when we treat opera as an English word meaning a long musical composition of vocal and orchestral parts, we pluralize it as operas.
Another word for prelude would be an overture or opening of an opera.
Relivesits actually reliefs (actually i doubted if relief had a plural...turns out it does)
Criterion is singular. The word criteria is actually the plural.
There is no plural form of the word Spaghetti.
Lorgnette is the alternative name for opera glasses
Actually, orifices.
There are numerous regional branches of Chinese opera, of which the Beijing 'Jingju' opera is one of the most notable.
The plural for algae is actually just algae
Supercilium is another word for eyebrow. The plural form is supercillia.
Book author is actually two words. The plural is book authors.
opera