"I don't know a thing about Fin.

"Well, there's a lot of snow," rescreening room.

"Well, there's a lot of snow," replied a wise guy.

Not a bad answer at that. "The White Reindeer," which will lead off a series on the Finnish cinema at American Film Institute Feb. 19 at 6:30, is full of snow: windblown vistas of snow, trees bent into bizarre shapes by snow, grim steppes glaring with snow, reindeer and people struggling knee-deep through snow, faces crusted with snow, huts and teepees buried in snow. It is an element, a given a ubiquitous fact of life for the Finns. and one cannot begin to understand them without it.

Even more than the Swedes, Finns live in a world dominated by weather, and one can sympathize with their desire to tear off all their clothes in summer. Judging by their films, their passions are direct and often violent. They drink their vodka from the bottle. They apparently couple with such frang dispatch that love scenes have to be cut short. They seem to dread these outbursts, too, which means that most of the time conduct their lives with massive calm. A fascinating people we know too little about.

"Reindeer" has the slow inevitability of a myth. It is about a beautiful witch who turns into a deer to lure men to their deaths. At times, she turns up with the prominent canines of a vampire, but mostly she lopes elegantly over the snow until her husband finally finishes her with an iron spear.

It is a rough film, and ponderous, but it stays with you not only for what it shows of the traditional life of the Laplanders but especially for its brooding power, like a Sibelius tone poem.

The three-hour epic, "The Unknown Soldier," which will be shown Feb. 20 at 9, is also slow and if anything even more powerful. An old-fashioned war movie (the kind we used to reenact in the vacant lot, scrambling out of foxholes and dying gorgeously), it depicts the life and death of recruits fighting that anticlimactic war against the Russians in 1941. Placed by fate on the side of the Nazis, the Finns didn't seem to have their heart in it as they had a few years earlier, in their astounding stand all alone against the Soviet divisions.

It is not a slick film. Its very gaucherie marks it with a deeply felt sincerity that is missing so often from our own war picture. The same honesty shines through "One Man's War," the modern story of a smalltime contractor struggling (and failing) against the big combines. The actors look like real people, and the sodden, gray landscape they move through is as depressingly authentic as any you will ever see.

Another side of Finnish cinema - which has been quietly productive for 70 years - is offered by Jorn Donner's "Black on White," a rather sour view (in color) of the modern urban public relations man and his affair with a girl at the office. Anticipating Bergman's Scenes from a Marriage" by five years, Donner exposes the hollowness of a model middle-class marriage, bringing to his story the visual sophistication that has made him famous as a Swedish director as well.

This series, the first of its kind ever shown in this country, also features pastoral dramas and period pieces. The schedule, following "Reindeer" and "Soldier":

Feb. 23, 9 p.m., "Redhead."

Feb. 25, 8:30, "Earth Is a Sinful Song."

Feb. 28, 8:30, "One Man's War."

March 1, 6:30, "Juha."

March 3, 6:30, "A Worker's Diary."

March 5, 9 p.m., "Black on White."

March 8, 9 p.m., "A Shot in the Factory."

March 10 6:30, "Harvest Month," 8:30, "The Man Who Couldn't Say No."