It’s fairly axiomatic that adjectives in English are always singular, even when the noun is plural. I don’t think I’ve ever heard a Romanian say “I have three greens shirts” or something.
Sometimes however adverbial phrases have nouns that are used as adjectives and there is a strong tendency for Romanian speakers to pluralize these. The simple rule of thumb is that if a word is describing another word, it is always singular in English.
What am I talking about? First, the nouns, which are plural in English:
- The tree is 100 years old.
- The book has 300 pages.
- The baby is 2 months old.
- The shirt costs 50 dollars.
- The movie is 60 minutes long.
However when these nouns are used in adverbial phrases to describe another noun, they take the singular form:
- It is a 100 year old tree.
- It is a 300 page book.
- It is a 2 month old baby.
- It is a 50 dollar shirt.
- It is a 60 minute movie.
As you can see, when one noun (year/page/month, etc) describes another noun (tree, book, page, etc) then the describing noun is singular.
In absolutely correct English, usually these “nouns as modifiers” are hyphenated, especially when it comes to ages. Therefore the most precise form of English is as follows:
- The man is 25 years old.
- He is a 25-year-old man.
Most native speakers of English are terrible at writing however so be aware that you won’t often see correct syntax. Regardless of how it is written, any time a noun is used to describe another noun, the describing nouns are singular even when the subject is plural.
- The girls in the group are are all 20 years old.
- It is a group of 20-year-old girls.
- My shelf is full of books that are 500 pages long.
- My shelf is full of 500-page books.
In Romanian, of course, adjectives always match the nouns in both gender and number but are never used quite in this fashion as adverbial phrases. In that sense, the grammar in Romanian is thankfully a little simpler than English.
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AND NOW YOU KNOW!