LEAD: It is the coldest fact of a film critic's life: movies may be art or escapism, but they are always consumer products.

It is the coldest fact of a film critic's life: movies may be art or escapism, but they are always consumer products.

If you forget that for a moment -put on your auteur-colored glasses, dismiss the schlock with a wave of your hand - some deceptive trailer, or preview, will knock you to your senses. It will remind you that films depend on marketing and that truth in advertising is not exactly a hot movie-biz concept.

The trailer for ''My Life as a Dog'' is a blatant reminder of how deceptive film promotions have become. In this sad, funny Swedish film, a 10-year-old boy is sent to relatives while his mother is dying. The preview shows endearing scenes from the movie - the young hero hugs his mom or splashes milk in his own face - while a voice-over intones as if inviting you to church: ''It was a year of innocence,'' a time of ''major heartbreaks'' and ''minor miracles.''

Now, ''My Life as a Dog'' is not that grandiose or false. But while I was scoffing at the trailer-makers' sentimental distortion, the clue to their deception suddenly appeared. ''Woof!'' said the little boy, on all fours. I realized that this international dog sound was the only word heard from the film. An unsuspecting viewer would never guess the movie is in Swedish with English subtitles.

And if anyone thinks the trailer-makers are even now slapping their heads and saying, ''Whoops! We forgot it was in Swedish!'' or ''Everyone knows it's in Swedish!,'' consider what care it took to eliminate all sounds except ''Woof!''

On a second viewing - I didn't believe my ears the first time - I strained to hear any other authentic word from the film, and caught only one: ''Ingemar.'' I assure you, this dangerously Swedish-sounding name was much fainter than ''Woof!'' Voice levels are no accident in films; and it is no coincidence that foreign-language films draw smaller audiences than English-language movies.

On the same bill with this non-Swedish trailer was one promoting ''Babette's Feast,'' the Danish film that won this year's Academy Award for Best Foreign Language Film. Even before winning the Oscar, the trailer announced its nomination in the foreign language category. Yet the preview eliminated motion as well as subtitles and sound from the film. Instead, there are stills from the movie and a narrator quoting reviewers' praise. Marketers seem terrorized by people who can't read and watch movies at the same time.

Mainstream commercial films are also resorting to sneaky suggestions. The frenetic television ads for the short-lived ''Switching Channels'' -Kathleen Turner in a rage in a television newsroom, facing off against Burt Reynolds and Christopher Reeve - looked suspiciously like ads for the hit ''Broadcast News,'' in which Holly Hunter, William Hurt and Albert Brooks race around a television newsroom. ''Switching Channels,'' a lackluster remake of ''The Front Page,'' bears only the slightest resemblance - a love triangle and a newsroom setting - to the more sophisticated ''Broadcast News.''

It is disturbing to see fanciful marketing applied to mainstream movies, for such exaggerations have usually been reserved for cheap horror films like the recent ''Prison.'' In its ads, inmates seem to run from some off-screen monster. But as it turns out, the blue light that seems to portend the ghost is the ghost.

The problem goes beyond the real irritation of wasting $6 or $7 and a few hours of time. Films that are advertised badly may never get a chance to reach their best audience. Wes Craven, of ''Nightmare on Elm Street'' fame, has made two stylish movies that marketers have dressed down to look like their slice-and-dice cousins. Mr. Craven aimed his most recent film, ''The Serpent and the Rainbow,'' beyond horror fans. Its hero is a scientist threatened not only by Haitian voodoo priests but also by the Tontons Macoute. Yet the television blitz showed the zombie hero screaming: ''Don't bury me! I'm not dead!'' The film is hardly a political treatise, but it's more thoughtful and balanced than that.