être + noun (no article)

Sitesurf 

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A few nouns describing people can also work the same way: elle est témoin de..., il est victime de..., ils sont élèves..., elles sont championnes...

https://www.duolingo.com/comment/14219021$comment_id=14222337

Sitesurf 

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"nous sommes frères" and "ils sont frères" are identical in construction, where "frères" works as an adjective.

"nous sommes des frères" is not impossible, "frères" is a noun and "des" is the plural indefinite article. In singular, you can say "je suis un frère de XX", meaning that XX has at least one other brother.

"ils sont des frères" is grammatically incorrect, because "ils sont + modified noun" automatically turns to "ce sont + modified noun". (Reminder: a modifier can be an article, a demonstrative or possessive adjective, a number).

"Ce sont frères" does not work either and this is why I pointed to "ils sont frères" or "ce sont des frères" and not "ils sont des frères".

These constructions are specific to the verb "être", so they do not apply to the verb "avoir": j'ai un frère, j'ai des frères.

I gave you other nouns acting as adjectives on another thread, but again this is unique to the verb "être" (or "devenir" or "rester"): un témoin, une victime, un(e) élève, un champion...

https://www.duolingo.com/comment/14220971$comment_id=14222310

Noun + de + noun (no article)

A "noun of noun" case happens when the second noun gives some information on the first noun's content, material or purpose:

• content: une bouteille de lait = a bottle of milk
• material: un pont de pierre = a stone bridge
• purpose: un couteau de cuisine = a kitchen knife.

As you can see, this French construction does not exactly match English where a noun can become an adjective if you place it in front of another noun. In French, you cannot use a noun as an adjective.

Written by Sitesurf@https://www.duolingo.com/comment/14079873$comment_id=14082490