When the producers of ''Quest for Fire'' first approached me, they suggested, having done some philological homework, that something approaching the old Indo-European tongue should be used. But at the time when man discovered fire, Indo-European had not yet come into existence. We are concerned with a very remote past -500,000 years ago - but a past that is localized. ''Quest for Fire'' is set in the ancestral land mass of the Europeans, or the Indo-European speakers. This meant that it was reasonable to construct a language having more in common wih Indo-European than with, say, Chinese or Indonesian. Even the linguistically unsophisticated are aware that English and Russian are practically the same language (both having once been Indo-European) when compared with any of the languages of the Far East. Consider Malay (or Bahasa, as it is now called) and you will see that it functions quite differently from English. Saya benchi dia means ''I hate him,'' and ''Dia benchi saya means ''He hates me.'' The words don't change their forms, only their positions. English is typically Indo-European in that it relies on the inflection of a pronoun to make its points. ''Him I hate'' and ''Me he hates'' are a little eccentric, perhaps, but word order is less important than word form. Latin didn't give a damn about word order. It didn't have to, since the ending of a word showed exactly what was happening. You can write, Puella puerum amat or Amat puerum puella or Puerum puella amat and it always means, ''The girl loves the boy.'' So the language I was to construct had to behave like Latin or Sanskrit. It had to sound as though it might one day turn into Indo-European. People usually expect what is called a primitive language to be simple, but the further back you go in the study of language the more complications you find. Simplicity is the fruit of the ability to generalize, and primitive man found it hard to generalize: One word for this man's weapon and another word for that man's weapon, but no word for weapon. It would have been stupid, in preparing a script in a new tongue for actors to learn, to be too pedantic about the probable complexity of an ancient language, so I compromised. But I could not compromise too much. If ''xxx'' means ''man'', then ''xxxe'' must mean ''to a man'', and ''xxxis'' ''by, with, or from men.''

Structure was not a problem, but vocabulary was. Take the very basic word ''fire.'' What was I to call it? Our modern Indo-European languages don't help us to dig out an original Indo-European root, much less suggest something older than Indo-European. Spanish has fuego, Italian fuoco, Portuguese fogo and Rumanian foc, all of which clearly derive from the Latin focus. But the Latin word for fire was ignis; focus meant the fireplace, the place where all gather to be warm, the living center of a room (you can see how it took on its modern English meaning). To call a fire a fireplace argues a kind of ancient taboo on certain words, like fire, that they are not spoken directly but instead connected with other, similar sources; it is like calling the toilet or jakes the bathroom. If German has Feuer and Dutch has vuur, the Scandinavian tongues have eld and ild. Polish has ogien and Czech ohen and Russian ogon and Serbo-Croatian vatra. Modern Greek has fotia and Yiddish (an Indo-European language, derivative of German, though spoken by a people whose ancestral word for fire is aysh) has sreyfe.

There is no community of form, which seems to argue that the Indo-Europeans threw up many words for fire, among which a variety of choices have been made by the languages which derive from Indo-European. I decided, somewhat arbitrarily, to have my fire-seekers call the hot bright magic atr. This suggests a taboo if we take it that atr could be the ancestor of the Latin ater/atris, which means black. The atrium was the open main court of a Roman house, so called because it was blackened by smoke from the hearth. Having chosen atr, I had to regard it not as a word but as a root -properly atr-. If I see the fire it is atrom, if the fire sees me it is atra, if I am surrounded by many fires these are atrois. Very Indo-European - or shall we say very un-Chinese, very un-Malay.

There was once a linguistic theory -put out by the German philologist Max Muller, and sometimes called the ''bowwow'' concept - which stated that words originally all tried to imitate the things they described. If a child calls a dog a ''bowwow,'' he is fulfilling this theory, just as when he calls a railway train a ''puff-puff'' or a ''choo-choo''. But the great Swiss linguist Ferdinand de Sassaure said that words were very arbitrary structures, and it is true that the imitative instinct is satisfied by very few words in any lan-guage. Moreover, if words are supposed to imitate sounds, there is a comparatively limited field of referents for the linguistic instinct to process. The things we see are regarded as more important than the things we hear. Nevertheless, the imitative instinct seems to be at work in a word like ''little,'' where the tongue creeps as high as it can in the mouth, making the littlest possible space between tongue and palate. When people roguishly say ''leetle,'' they are decreasing the space even further. Let us look at the word for moon in 25 languages: French: lune Spanish: luna Italian: luna Portuguese: lua Rumanian: luna German: Mond Dutch: maan Swedish: mane Danish: mane Norwegian: mane Polish: ksiezyc Czech: mesic Serbo-Croatian: mesec Hungarian: hold Finnish: kuu Turkish: ay Indonesian: bulan Esperanto: luno Russian: luna Greek: fegari Arabic: qamar Hebrew: levanah Yiddish: levoneh Japanese: tsuki Swahili: mwezi

What you would expect from a good descriptive word for moon is an attempt on the part of the mouth to imitate both its roundness and its elevation. The Latin languages and the Germanic languages and the tongue of the Malay archipelago agree on this. The lips are round and the back of the tongue is high in the mouth. Polish seems to make a terrible hash of the moon, while Mother Russian (luna) very sensibly keeps close to the Latin form (luna). The Arabic (and Maltese) word turns the moon into a rasping scimitar. Hebrew, which Yiddish follows, has a word which suggests a ritual involving the moon rather than the moon itself. I had the task, in fitting ''Quest for Fire'' with a soundtrack, of making a plausible pre-Indo-European word. It has to have a long u in it, so that the meaning of a word like ''moon'' can be discerned not only by the literal meaning but by the round, moonlike shape that one's mouth makes when pronouncing the word, and there is a majority acceptance of a nasal sound after it, so why not something with a buuuun-root? I love the moon: Buuuunan; The moon loves me: Buuuunu; I am surrounded by moons: Buuuunois.