Why is the word “the” declining in English?

The drop is indeed puzzling, but unlike Brian Collins I don’t think it reflects an actual change in English usage (such as the perishing of the encyclopaedic the—that wouldn’t make that much of a dint). I also don’t think Second Language Learner English would make such a dint. It’s about the representation of texts in the Google n-gram corpus.

The dip seems to be from 1960 on. What I think is happening is more texts from then on in the Google corpus have headline-like English, of the kind you see in dot points; headline-like English drops the readily. It’s a real written register (in business reports, instructional manuals, &c), and it’s not one that was really around before then. So:

  • “Attach nozzle to faucet, see diagram A”
  • “Referred director to board for more information”
  • “Computer incapable of processing input”

EDIT: See comments: this is unlikely to account for the drop either, especially in fiction.

Linguistics: Why do interjections differ?

Because, contrary to what you might think, interjections are not always pure spontaneous exclamations from deep in the neural cortex, that are universal to all humans.

A few are; as I noted in Nick Nicholas’ answer to Are there any short expletives that sound the same in different languages?

Nick Enfield [Page on sydney.edu.au] (who I did linguistics with, and boy does he look different twenty years on) just got an Ig Noble [Improbable Research] for claiming the universality of Huh? (The Syllable Everyone Recognizes, Is ‘Huh?’ a universal word?)

Of course the realisation of Huh? does differ by language; in the Mediterranean, for example, it is E? But the general idea is a mid vowel (as close to a schwa as your language allows), with a questioning tone.

However plenty of them are culture specific; they may not be arbitrary in themselves, but the choice of which interjection to use can be; and in fact interjections can be borrowed between languages, just like any other word.

Two instances from Modern Greek.

  • “Ouch” in Greek is traditionally [ax, ox]. English [autʃ ~ auts] has now been borrowed into young people’s Greek, from TV.
  • The Greek sneeze interjection is [apsu]. I’ve just discovered that the Turkish interjection is [hapʃuː], and [apsu] is just [hapʃuː] nativised to Greek phonology. (How is the sound of a sneeze written out in different languages?)

Does the Greek word for Palaces, Megara, come from the Aramean word Magharat or Zagharat “caves”?

Maybe.

There is a plural megara word in ancient Greek, which means “a kind of crypt into which live pigs were thrown during the Thesmophoria festival”. This is related by both Chantraine and Frisk to Hebrew me‘ārā “cavern”, meaning it is Semitic (in all likelihood), and thus related to Arabic Magharat.

The singular megaron “hall” is less definite; it may be related to the town of Megara; it may be related to megas “big”; it may be borrowed from an external language (Frisk rejects the proposed Indo-European derivation by Brugman), and Chantraine notes that a Condoléon thought it was indeed the same word as the plural megara “crypts”. But that’s just one authority relating the two words.

Zagori OTOH is pretty obviously derived from Slavonic Zagore, “beyond the mountain”.

Do some incorrect or imprecise terms stick just because English language hasn’t better options?

Never, never, ever underestimate the power of inertia.

In the instance you cite, of sex addiction vs compulsivity: the distinction is itself fairly new, and the use of the description to describe the patient has not yet stabilised, because the notion of compulsion as a medical condition has not been pervasive. So there’s a huge amount of inertia behind addiction, and an even huger amount of inertia because there hasn’t been until now a term for “one suffering from a compulsion”, to match “addict = one suffering from an addiction”.

If we went to Latin, we would use the past passive participle, find it to be compulsus (cf. addictus), and say that the person is a *compulse. But that hasn’t happened in English with any of the -pulse/-pel verbs. Not least because pulse and impulse as nouns are abstractions.

Since a sufferer of compulsion is grammatically one who is compelled, we could use *compelee. But compel and compulsion have actually diverged—compel is not used in the psychological context.

So, by accident, we don’t have a straightforward derivative word to describe such a patient. What to do, OP, what to do…

… actually this has already been solved. compulsive can be used as a noun to describe someone who exhibits a compulsive disorder: a sex compulsive. This is also something that English does with adjectives; cf. captive prisoner > captive. It sounds odd to us, because compulsive disorder describes the compeller and not the compellee; but it’s better than the alternatives, and it’s already in use.

The grammatical strangeness may slow down the take-up of compulsive; but if there is a compelling (ha!) case for a single word to be used for sufferers of compulsion, it will be taken up anyway.

Are there certain types of words that humans remember far easier than others?

Shulamit Widawsky is right about the emotive loading of words affecting their memorability.

In the specific context of dirty words, you may well have been highly motivated to learn them. (There’s always keen motivation to learn dirty words in foreign languages, as evidenced here on Quora.) If you were strongly motivated and were delighted by the frisson of taboo, then the words are likelier to have stuck.

Does that mean naughty words should be prioritised in language textbooks? Well, if you’re taught them boringly by rote, all the fun goes out of them, and if all the fun goes out of them, you’re less likely to remember them. So in fact, maybe not.

I’d say getting people to look up their own words, as they need them in composition, can be very helpful, for a similar reason: it’s a word you needed, and invested some effort in. A lot of my German and Klingon stuck that way.