How serious your son is versus how serious is your son. Why the correct answer is not the second one?
French exclamatory expressions often use interrogative words, such as:
quel (what + noun)
que (how + adjective)
comme (which translates to how for exclamations)
In English when what you’re exclaming about is clear, you don’t need a noun in your exclamation and you can just say something like How beautiful!, skipping both a noun and the verb. To do the same thing in French, you can skip the noun, but you can’t skip the verb and the conjugation of être must be included. But you have a choice for the exclamatory expression: You can use que or comme, which are totally interchangeable, or sometimes skip the exclamative word and make a very short sentence with just an exclamation mark.
Que c’est beau! How beautiful!
Comme c’est beau! How beautiful!
C’est beau! That’s beautiful!
Quel oiseau magnifique! What a beautiful bird!
Que c’est laid! How ugly it is!
Comme il est mignon! How cute he is!
Quoi! Il a fait ça! What! He did that!
Qu’elle est douce, cette écharpe ! How soft this scarf is !
Qu’ils sont gentils, ces enfants ! How nice these children are !
One exercise in Duolingo is "quelle belle armoire", which doesnt have an article, what's with that?
To understand why quelle belle amoire doesn't have and article you need to be aware of the following three points.
Generally, nouns require a determiner.
There are various types of determiners, such as articles (le, la, des, etc.), possessive adjectives (mon, ton, etc.), demonstrative adjectives (ce, cette, etc.), cardinal numbers, exclamative adjectives, etc.
quelle can act as a exclamative adjective (as it is in the phrase quelle belle armoire - what a beautiful wardrobe)
How + verb "to be" + direct object is a question. How tall is he?
How + direct object + verb "to be" is an exclamation. How tall he is!
The French phrase is written as an exclamation. If it were a question, it would be "ton fils est-il sérieux ?" or "est-ce que ton fils est sérieux ?"
A translation of this exclamation in English is "how serious your son is!"
Well, I tried to have it feel more English by changing it to "your son is so serious!" which is more natural in English and still has the sense of it being an exclamation This was too idiomatic for DL. You never know. Awkward English vs. too idiomatic: toss a coin.
However, many of us use the exclamation form, "How serious is your son!" I'm surprised you have never heard this.
It's a little unusual to some people maybe, but not at all awkward or incorrect.