Everyone laughed when Fox Broadcasting started a fourth television network. Nobody laughed at its first attempts at comedy, however. "Mr. President" made "Mr. Belvedere" look like "Mister Roberts."

"Mr. President" was just one of the forgettable Fox flops. Does anyone remember "Boys Will Be Boys," "Karen's Song," "Women in Prison," "Down and Out in Beverly Hills" or "The New Adventures of Beans Baxter"? You have to have a masochistic streak to remember shows like those.But through trial and error, Fox found a nook. It has attracted a bubbling youthful audience with shows like "Married . . . with Children" and "21 Jump Street" and for the new fall season, it will capitalize on that target group.

Fox doesn't have as many stations as the big three networks do, nor reach as high a percentage of the viewing nation. By adding Monday night programming to its schedule, Fox increases its output to only three nights a week. But the demographic desirability of the Fox viewership can't be denied.

On a recent Sunday night, Fox came in third, not fourth, among the networks, beating ABC. One reason is that the lazy established networks continue to treat summer as a toxic dump site, programming almost nothing but ratty reruns, handing their audience over to whoever wants it.

This summer, Fox wants it. A new Fox show, the "Candid Camera" ripoff "Totally Hidden Video," earned horrible reviews and threats of a lawsuit from "Candid Camera" creator Allen Funt. But it also earned the highest ratings of any Fox premiere ever.

Among Fox shows for fall are a few with possibilities. "Alien Nation," premiering Monday, Sept. 18, is based on the entertaining semi-hit movie of the same name, a fanciful cop saga set in Los Angeles of 1995 (the movie took place in 1991). The city is host to an all-new ethnic minority: Newcomers, who arrived on a wayward space ship from another planet and are trying now to find a place in the melting pot.

Newcomers love to slurp sour milk right out of the carton. They don't have wavy hair so much as wavy heads.

The two main characters are buddy cops - one human, the other an alien. The human is a surly, loutish loud-mouth and the alien a sweet-tempered, sensitive soul. Hey, it sounds like Sam Donaldson and Diane Sawyer of ABC's "Prime Time Live"!

Getting a jump on the new fall season, which officially starts Sept. 17, Fox will premiere the sitcom "Open House" on Sunday, Aug. 27. This new show grew out of an old one: "Duet," a so-so comedy, now canceled, whose most winning character was Linda Phillips, a snide snob played with scintillating sizzle by Alison LaPlaca.

To judge from the pilot episode, however, the producers don't realize LaPlaca's appeal. They have made this another of those gabby ensemble comedies in which a gallery of characters each come forward to get a laugh (or not) and then recede.

Brooding youths have been a key ingredient of Fox's success. Young viewers seem to love the sullen, self-important mini-cops of "21 Jump Street." So Fox has spun off another series from that: "Booker," starring the swarthy and beefy Richard Grieco as yet another maverick, renegade flatfoot.

"Booker" premieres Sunday, Sept. 24, displacing "21 Jump Street" which moves to Monday nights on Sept. 18. And Fox has more brooding youths waiting in the wings. Francis Ford Coppola will produce six episodes of "The Outsiders," based on his movie version of an S.E. Hinton novel. This fits right into Fox's teens-in-jeans formula.

"Outsiders" is one of the back-up shows that Fox has on tap in case other shows bomb out. The other backups include "The Simpsons," a daft half-hour animated sitcom based on characters that have appeared in little blackouts on Fox's "Tracey Ullman Show."

"Ullman" and "It's Garry Shandling's Show," which air on Fox Sunday nights, are classy and clever productions. For the most part, though, Fox has found success by aiming low.

Clearly, Fox Television is not going to go broke overestimating the intelligence of the American people. But then, that's not the way the other three networks made their fortunes, either.