The following summaries of current, widely shown films are provided to help readers plan what to see. If additional coverage of a film has appeared in the Monitor, the date of the article is given in italics after the summary. Inclusion of a movie does not imply Monitor endorsement. The Movie Guide is scheduled to appear on the first and third Thursdays of the month.m
BACHELOR PARTY - There are even fewer laughs than you'd expect in this deliberately gross farce about a young man's prenuptial revels. Directed by Neal Israel. (Rated R; contains rough language and much sexual humor.)
BEST DEFENSE - A lazy defense engineer passes off someone else's brainstorm as his own, and finds himself in trouble with a demented black-marketeer as well as his own bosses. As directed by Willard Huyck, some scenes are quite amusing, others just coarse and stupid. (Rated R; contains sex and much vulgar language.)
THE BOSTONIANS - Literate, exquisitely filmed, deftly performed adaptation of Henry James's imposing novel about a 19th-century feminist and a chivalrous Southerner who compete for the heart and loyalty of a young woman involved with the fight for women's rights. There's plenty of room for argument over James's attitude toward ''the woman question'' and the reproduction of it in this film, but it's hard to fault the expert filmmaking of director James Ivory, writer Ruth Prawer Jhabvala, and producer Ismail Merchant, who have worked together for more than 20 years. (Not rated.) Aug. 2.
CANNONBALL RUN II - This farce has an idiot energy that almost prevents it from being completely unwatchable. The ramshackle story about a cross-country automobile race was directed by former stunt man Hal Needham. (Rated PG; contains vulgar language and coarse, often sexist humor.)
CAREFUL HE MIGHT HEAR YOU - A parentless little boy becomes a pawn in an emotional struggle between his two aunts in this well-made but stodgy Australian drama. Directed by Carl Schultz. (Not rated; contains strongly emotional scenes involving a child.)
CONAN THE DESTROYER - The hero's quest for a magical key is lightened by a little self-deprecatory humor, but most of the action is a dull mixture of sword-and-sorcery cliches. Directed by Richard Fleischer with a touch heavy enough to recall the prose style of Conan creator Robert E. Howard. (Rated PG; contains violence.)
DONALD DUCK FILM FESTIVAL - Not only entertaining, it's a study in animation styles of the '30s and '40s. But it would be stronger if more different examples replaced the long excerpts from ''Three Caballeros.'' The directors include Wilfred Jackson, Ben Sharpsteen, Jack King, Jack Hannah, and Norm Ferguson. (Rated G.)
ELECTRIC DREAMS - A computer and its owner compete for the affections of their neighbor, a cellist with a weakness for synthesized sounds. Mostly dull and foolish despite the visual energy of director Steve Barron. (Rated R; contains vulgar language.)
FIRST NAME CARMEN - Also known as ''Prenom: Carmen'' and loosely suggested by Bizet's opera, this is another pungent, provocative, proudly outrageous essay by director Jean-Luc Godard, who appears more interested in oblique musings on the state of sex and civilization than in his characters, a terrorist and her bewildered boyfriend. The melodramatic plot and sad, obsessive sexual encounters seem more elemental than exotic in their context of seascapes, cityscapes, and Beethoven quartet music. (Rated R; contains very graphic sex and some violence.)
THE FOURTH MAN - There's great visual imagination and very, very bad taste in this pitch-black comedy about a writer who becomes sexually obsessed with his girlfriend's other lover. Directed by Dutch filmmaker Paul Verhoeven. (Rated R; contains very graphic sex, violence, and perverse use of religious imagery.)
THE GODS MUST BE CRAZY - A charming, funny, dramatic, discumbobulated comedy shot in Botswana by director Jamie Uys, who also served as writer, producer, and editor. The story deals with the lives of a Kalahari tribe that has never heard of civilization, and goes on to include a slapstick romance, among many other unexpected things. (Rated PG; contains some violence.)
GREMLINS - At first there's only one, and he's very cute, but if you aren't careful he has zillions of babies that shift from mischievous to malicious. There's lots of fun and inventiveness to the tale, although director Joe Dante has trouble balancing the humor and horror, both of which are surprisingly blunt. (Rated PG; contains a little vulgar language and a lot of strong though cartoonlike violence.) June 7.
IMPROPER CONDUCT - Capably assembled documentary about controls on individual freedom in Cuba, emphasizing the oppression of homosexuals. Directed by Orlando Jimenez Leal and the great cinematographer Nestor Almendros. (Not rated; includes frank discussion of homosexuality.)
THE LAST STARFIGHTER - An ordinary teen-ager is recruited to help win an interstellar war in another galaxy. The story is engaging and the visual effects are terrific in a Steven Spielberg sort of way, but the pace is hurried and the climaxes don't generate much emotion. (Rated PG; contains cartoonlike violence.) July 19.
THE MUPPETS TAKE MANHATTAN - And the place will never be the same after Kermit's big show opens on Broadway and Miss Piggy demolishes a Central Park mugger. The rest of the gang is in top form, too, and under Frank Oz's direction the action is fast and funny for viewers of all ages. (Rated G.). July 19.
THE NEVERENDING STORY - A book-loving boy is literally drawn into the story he's reading, about a magical kingdom that's threatened by a menace called the Nothing. Directed with great visual imagination by Wolfgang Petersen, though the effects aren't consistently realistic, and the screenplay has flat moments. (Rated PG; contains cartoonish violence and some surprisingly intense emotion.) July 19.
PHAR LAP - Involving drama based on the true story of an extraordinary Australian racehorse that became a national icon during the early years of the depression, then died under mysterious circumstances. The most expensive Australian production to date, the picture was carefully directed by Simon Wincer, who looks at both the heroic and seamy sides of racing. (Rated PG; contains a little vulgar language.)
THE POPE OF GREENWICH VILLAGE - Some energetic performances are the only notable virtues of this melodrama about two small-timers who pull off a heist and find themselves in trouble with the mob. Raggedly directed by Stuart Rosenberg. (Rated R; contains much vulgar language and some very harrowing violence.)
REVENGE OF THE NERDS - There are few surprises amid the coarse jokes and predictable plot twists, as a bunch of intellectuals strike back at the jocks who make their first college days miserable. But the underlying intelligence of director Jeff Kanew glimmers through at times, and is reflected in a couple of nice performances. (Rated R; contains nudity and much vulgar language.)
TOP SECRET! - There's less of Karl Marx than the Marx Brothers in East Germany, according to this crazy comedy about an American rock star who finds intrigue, romance, and silliness during a goodwill visit. Directed by Jim Abrahams, David Zucker, and Jerry Zucker, whose sight gags were much funnier and more varied in their earlier ''Airplane.'' (Rated PG; contains some surprisingly coarse and kinky humor.)
UNDER THE VOLCANO - Serious adaptation of Malcolm Lowry's ambitious novel about an alcoholic struggling to have a few decent moments on the last day of his disappointing life. Sensitively filmed by John Huston and superbly acted by Albert Finney. (Rated R; contains much drinking and a sordid brothel scene.) July 5.