Because the answer given is wrong. “Loyal” is certainly an adjective. I’d have called “the” an article, myself, rather than an adjective, though I suppose technically it supplies further information about the noun.
I seem to remember something about when analyzing a sentence, first remove all the prepositional phrases (“to loyal friends” and “of the family”) Then you’re left with the basic sentence “He sent twenty letters.” HOWEVER, that would also eliminate the word “the”.
Adjectives tell A) what kind? B) which one(s)? C) How many?
“The” pen implies a specific pen, “which one?”
“A” pen implies one, “how many?”
And yes, “loyal” tells what kind of friends and is an adjective. If you want to get even more technical, “of the family” is a prepositional phrase acting as an adjective, describing “friends” because it defines which ones. “to loyal friends” is also a prepositonal phrase but it acts as an adverb, describing “sent” (tells where).
No, they’re not. Articles belong to a different syntactic class than adjectives. Unfortunately, I don’t have my grammar book handy, but there are two obvious differences that are observable right away and which demonstrate a clear distinction between these categories.
First, adjectives that are modifying a noun are mostly interchangeable with regard to word order. It’s possible to say: “the resourceful, clever man” or “the clever, resourceful man”. It doesn’t matter which comes first. But the word order with articles are much, much stricter. It is grammatical to say “the clever man”, but “clever the man” is not correct. The article must begin the noun phrase. And it’s similarly impossible to use two articles together, “a the man” is not grammatical, whereas adjectives can be stacked together indefinitely. There’s no limit.
Second, adjectives can generally be modified for comparative and superlative forms. There’s not just “big”, there’s also “bigger” and “biggest”. There’s not just “interesting”, there’s also “more interesting” and “most interesting”. This is impossible to do with “the”. (I mean, seriously, try it: the’er? the’est? In a sentence: “He’s the’est man I know!”)
People need to get away from the idea that any word that modifies a noun must be an adjective, because that is simply incorrect. Nouns can also modify nouns, as in the case of “London fog”. Grammar is more complicated than the amateurs would have you believe (and this includes the people in charge of the website for the University of Ottawa’s Writing Centre). It doesn’t help us untangle these puzzles when we summarily label everything in front of a noun “adjective”, because we then miss the many subtleties involved.
I should’ve added originally, to answer the OP, that “twenty” is also not an adjective, as is partially demonstated by CookingWithGas’s joke. It is, like “the”, a determiner, specifically a quantifier. As with the definite or indefinite articles, cardinal numbers cannot be repeated. It is ungrammatical to say * “There were two fifteen drunken freshmen in the room.”. And I believe that there are special word order considerations with quantifiers as well, though I can’t remember off the top of my head. This means that the only genuine adjective in the OP’s sentence was “loyal”.
The dictionary says otherwise, but the dictionary is for definitions, not grammar. Every single dictionary in print is wrong when it comes to grammatical terminology. Which says just about everything you need to know about the prestige of linguistics as a scientific discipline. But to try to look on the bright side of things, it is not at all uncommon when lay audiences try to employ precise technical words that they “misuse” the words in a sense which is unacceptable in scientific circles (e.g. the prevalence of the term “escape velocity” in casual usage when there is no such thing - it’s “escape speed” because it’s not a vector).
For this reason, I’m not really opposed, per se, to us accepting “twenty” as an adjective. But if we want to be precise, if we want “adjective” to actually mean something when we use the word, just as we want in physics to differentiate between a scalar (with just magnitude) and a vector (with both magnitude and direction), then we need to figure out what this crazy thing called “adjective” actually entails. And linguists have done this. “Twenty” just doesn’t make the cut. It is a determiner, not an adjective.
I’ve no specialism in linguistics myself, just English language in general. But if the debate is “is the an adjective” then I’ll say no - it makes no sense to consider determiners to be adjectives. As a non-linguist, my reasoning is likely to be unsatisfactory to most, I suppose. I’ll try and explain if anyone cares
The definition of an adjective, from The Cambridge Encyclopedia of the English Language (David Crystal, 2nd ed., p. 211): “Words which express some feature or quality of noun or pronoun.” He goes on to say: “To count as an adjective, a word must be able to function in both attributive and predicative positions.”
Attributive: “a big house.”
Predicative: “The house was big”.
He also states that "The adjective is a good example of a word class with fuzzy edges…Numerals…share some of the properties of central adjectives, but not others. They can occur before a noun and after be (the four cats, she’s four), but cannot compare or take -ly.
And on the subject of determiners: They are “a group of words which can be used instead of the and a in the noun phrase…Examples include some, much, that.” Articles (a, an, the) do not get their own section, but at no point are they mentioned as adjectives either. I hope this is somewhat useful. The text I’m quoting from was used in my Historical Linguistics class, and it’s published by Cambridge UP.
So: the is definitely not an adjective. Loyal certainly is. For the purposes of K-12 students, twenty is an adjective. A linguist might disagree. IANALinguist by profession, but I minored in Linguistic Studies, so I have an idea of what I’m talking about. Hope this helps rather than bores.