"Is this country's coast short?"
Translation:هَل ساحِل هٰذا الْبَلَد قَصير؟
If you notice in English, (this) is pointing to (country's). This is pointing to the country. In your sentence, you are pointing to the Coast ساحل (and somewhat the order is not correct). So, the proper sentence here (as translating from the English one) should be ساحل هذا البلد قصير; where the pointing article هذا (this) is pointing to البلد (the country), as you would say in English the coast of THIS country is short.
Well, the relationship here between ساحل and بلد is a Genitive relationship (or as some books call it OF-relationship). Which simply means, it is X of Y type of relationships between two nouns.
So, originally, a coast of a country (everything indefinite here), would be ساحلُ بلدٍ (sáHilu baladin), the word بلدٍ here is said to be in Genitive (of a country).
Now, let's say we want this to be DEFINED. In English, you would say The coast of The country. In Arabic, the definite article is added ONLY to the second term in the compound: ساحلُ البلدِ (sáHilu al-baladi), and notice how the Tanwin in (balad-in) was removed and exchanged for simple (-i) to become (al-baladi); Tanwin can be considered like a sign of indefinite noun (like a/an in English but it's added to the end of the word).
So, this is the basic structure: The definite article (AL) is added only to the second term in the Genitive relation.
If we want to add (this), we have to bear in mind that (this) is actually equal in some way to (the), because it defines the noun it comes before it (in English or Arabic). So, the rule for adding (AL) in the Genitive relation as I've mentioned before, is the same for هذا (this); It is added only to the second term in the Genitive relation. Except that in Arabic, we don't remove (AL) when we add هذا (while in English you have to remove THE and put THIS).
In your sentence, you've defined the first term in the Genitive relation, and in Arabic this is not possible; Kind of, doesn't make sense really. However, I can fix your sentence simply by adding في (in) to break the bond between ساحل and هذا البلد. And your sentence would be هل الساحل في هذا البلد قصير؟ which translates to: Is THE coast IN this country short?
Which, of course, gives the same meaning really but without any Genitive relationships between the two words. But I doubt Duolingo would accept it that way.
Man, you're a life saver. Thank you so much, again! I think I get it now. The genitive case remains as is i.e. the coast of the country ساحلُ البلدُ Only that the هذا is put in between the two, i.e. before البلد to point towards the country. Right? And I get why we keep both هذا and ال, unlike english. Its kinda like Turkish.
It can be so and it's no different than the original English sentence stated here (I guess Duolingo still doesn't recognize this?).
Your sentence here would translate to: Is this country a short coast?
The word ساحل (saaHil) should come before (hadha al-balad) and as such the phrase will be (coast of this country) ساحل هذا البلد.
Unlike English, in Arabic, when we have a genitive relation between 2 nouns, the definite article goes to the second term in the compound. Thus, we have بلد (balad: country) and ساحل (sáHil: coast), if combined to make (the coast of the country) it would become: ساحل البلد (sáHilu al-balad) or more phonetically (sáHilul-balad). Adding هذا (háTHá: This) before (Al-Balad) won't change anything in fact, it's just an additive.