When the major-league baseball season began this week, it marked the start of six months of drama and romance for lovers of the national pastime. But for another segment of the population, it was the beginning of six months of degeneracy.

People who bet on baseball are a special breed. While other sports attract large numbers of casual gamblers, baseball appeals to the hard core. They can stay in action day and night throughout a season, spending most of their waking hours poring over statistics or twisting their radio dials to pick up any game they can.

One reason for the limited gambling appeal of baseball is relative complexity of betting it. Most of America now knows what it means when the Cowboys are established as a six-point favorite over the Redskins. Fewer understand when the Orioles are listed as 5 1/2 - 6 1/2 favorites over the White Sox.

For the uninitiated, baseball bets are made in units of $5, and there is a 5 implied in quotations of baseball odds. If you like the Orioles over the White Sox, you lay odds of 6 1/2 to 5. If you like the White Sox, you bet $5 to win $5.50.

To a neophyte, this may sound very beguiling. You don't have to worry about point spreads, as in football or basketball; all you have to do is pick winners. How easy!

The most astute baseball gambler I know, whom I'll call Harry, maintains that betting on obvious, standout favorites is what ruins most of his brethren. t

"When you start playing favorites and laying 8 to 5 and 9 to 5, a few losers will put you into concrete," Harry said. "The key to this game is to look to get a price."

Harry looks to bet underdogs in certain types of situations. He likes to identify, and bet, good pitchers on weak teams. (One of his favorites is Rick Langford of Oakland.) And he likes to spot situations when good teams will be most vulnerable.

The Red Sox and the Astros, for example, are much less effective on the road than they are at home. The slow-footed Yankees are at a disadvantage on natural grass.Certain team's effectiveness will vary greatly against left-and right-handed pitchers. Montreals lineup, laden with right-handed power, will have difficulties against average right-handed pitchers, but may tee off against a good left-hander like the Phillies's Steve Carlton, who faces them tonight.

As important as these special situations are, there is an even more crucial, more fundamental, more obvious factor in betting baseball. That is to identify teams that are in long-term winning or losing trends.

Thomas Boswell, the Post's baseball writer maintains that the game has increasingly become one of lengthy streaks.

"They can last half of a season," he said. "Last year the Orioles had a stretch of 100 games where they played .730 ball. The Rangers had a post-All-Star Game slump where they lost 30 or 40. You should get on and stay with it until you're convinced the trend has turned."

Boswell maintains, further, that some trends extend for years; when a team starts to decline, that decline will be almost irreversible unless the club makes major personnel changes. Accordingly, he believes that all of the fallen giants of 1978 -- the Yankees, Red Sox, Royals, Dodgers and Phillies -- are in permanent down trends.

He adds, "Once a team that was thought to be a contender goes sour, all the theories about complacent rich ball players come into play. One team that this could happen to is the Red Sox. If they have some early disappointments, they could fold their tent by midseason, and they'd be a good team to bet against."

Boswell, it should be noted, does not bet. He loves the game so much that he feels his esthetic appreciation of it might be spoiled if he had money at stake.

How pure can you get?