A few years back an English television writer came up with an idea for a series about a newly divorced man trying to make it in the world of the suddenly single. He wanted the show to look "American," and so he wrote it with a specific American actor in mind as the lead: Judd Hirsch.

This season Ed. Weinberger Productions ("The Cosby Show," "Amen," "Taxi") is doing an American version of that Americanized British sitcom. It's called Dear John (8 p.m., Ch. 2), and you'll never guess who the star is.Oh. You guessed.

Hirsch, who won audience affection and two Emmys for his performance as compassionate cabbie Alex Rieger on "Taxi," plays John Lacey, a New York teacher whose wife has left him for another man - who also happens to be John's best friend. But this show isn't about John's ex-wife and his ex-best friend. It's about the unusual group of characters he meets at the community singles group he turns to for support.

The group, called the One-to-One Club, provides "Dear John" with a "Taxi"-like ensemble: an attractive, quick-witted-but-insecure redhead (played by Isabella Hofmann); a macho smoothie (Jere Burns) who sees the group as a good place to meet "fems, dames, broadskies"; a meek introvert (Harry Groener) whose Bulgarian wife left him during their wedding reception; and a saucy group leader (Jane Carr) who asks every suffering soul if there were any "sexual problems." (Which reminds me: That subject - sex - comes up a lot. This is definitely an adult-oriented comedy.)

Hirsch plays the title role like it was written for him - which, of course, it was. He's the Everyman through whom the rest of us can view the screwball world he lives in. Hofmann has a nice touch with both straight lines and punch lines, giving the show much the same dimension as Marilu Henner gave "Taxi." And Burns and Carr both have breakout potential; if the show catches on you'll be seeing imitations of "Kirk" and "Louise" at a party near you.

The writing in the pilot seemed a little tired, like maybe they were trying too hard to make "Taxi" again. But the script is strong enough that I found myself laughing out loud several times - something I rarely do with sitcoms. And the characters are delightful. With its sweet time period, this show should evolve into one of the season's brightest new hits.

Even if it isn't really new.

* ALSO ON THE TUBE TONIGHT: As you've probably already heard, the big change in store for The Cosby Show (7 p.m., Ch. 2) this season is supposed to be the return of Lisa Bonet, who is dropping out of Hillman College - and "A Different World" - to return to the Huxtable homestead. But based on an advance screening of tonight's season opener, there's an even more drastic difference: The show isn't funny anymore.

I know, I know - we're into a lot of exposition to wade through in the episode: explaining why Denise wants to leave school, setting up Theo's first year at NYU (which he messes up immediately) and introducing us to Vanessa's (Tempestt Bledsoe) new 'do (which looks a little like an atomic bomb - after the explosion) and the loose fitting tops Denise will be wearing - a feeble attempt to hide Bonet's pregnancy.

But "Cosby" has always been able to cover a lot of ground without sacrificing humor in the past, and this time the comedy isn't there. Instead, there is an over-riding feeling of grim self-consciousness - forced funniness, not the natural, joyful humor the show has earned its reputation with. Let's hope this is just a one-shot problem and not the beginning of the end for one of TV's all-time sitcom classics.

Speaking of A Different World (and we were, weren't we?), the "Cosby" spinoff returns for its second season tonight at 7:30 sans the "Cosby" connection - Bonet. Also gone from the cast is Marisa Tomei, the show's one redeeming feature last fall. Debbie Allen ("Fame") has taken over as producer-director. Whether or not she'll be able to turn this into a show that justifies its high ratings remains to be seen.

Later on NBC, Jay Leno, Garry Shandling and David Letterman turn out to help Johnny Carson celebrate The Tonight Show 26th Anniversary (8:30 p.m., Ch. 2). And how has Carson been able to last so long as the king of late night television? Says Letterman: "Part of his success is certainly due to the fact that, for the past seven years, he hasn't had to worry about being upstaged by the show that follows him."

Meanwhile over on CBS, Anne Archer and Sam Neill star in Leap of Faith (8 p.m., Ch. 5), a new made-for-TV movie based on the true story of a woman who "heals" the cancer in her body through a psychological exercise called "visualization." The movie is well-made and persuasive - a little too much so, I'm afraid. While I'm sure our minds have vast powers that few of us understand or appreciate, I'm a little concerned about a movie that may persuade viewers to abandon prudent medical treatment and turn to mental exercises as a last hope of healing - on-air disclaimers to the contrary. Still, the movie is interesting and may well provide useful information, as long as viewers realize that the happy ending here may not come about in every case.

Elsewhere: The A's take on the Red Sox in Game Two of The American League Championship Series (6 p.m., Ch. 4); 48 Hours (7 p.m., Ch. 5) reports on the steps some couples are taking to combat infertility; KBYU premieres The American Experience (8 p.m., Ch. 11); KSTU starts building a fire for its new syndicated series with the 1953 film version of H.G. Wells' War of the Worlds (7 p.m., Ch. 13) starring Gene Barry; and Ronald Reagan is The Gipper in the colorized version of the 1940 football classic, Knute Rockne, All American (8:20 p.m., TBS).