“The Fundamentals of Caring” is a PG or PG-13 dramedy straining to earn an R. Lightweight, with heavy underpinnings, it wins a laugh here and there and occasionally rises to “cute.”

But if you want an example of a script whose author doesn’t know tone, how to match the subject to the characters and make the story connect to its audience, this is it.

Writer-director Rob Burnett, freed from his “Late Show with David Letterman” duties, underscores every F-bomb in the screenplay. He litters the screen with tone-deaf profanities even as he reaches for the heartstrings or tries to wring maximum laughs from a featherweight road comedy about a heavy-hearted caregiver trying to brighten the statistically-short life of a teen with muscular dystrophy in his care.

Yes, it’s “Me Before You” without the overwhelming wealth, the cynicism or the silly love story.

Paul Rudd is Ben, a sad-faced novelist who has “retired” from that and taken up the one thing he figures he can still do for a living — caregiving. He’s had the six week course, and learned, as the film’s title suggests, “the fundamentals.”

Remember “ALOHA,” his instructor lectures. “Ask, Listen, Observe, Help, Ask Again.” He’s memorized the “commandments” of this important, overlooked livelihood, or at least, he should have.

“I cannot  take care of another until I first take care of myself.”

His first-ever suburban Washington state job is given, reluctantly, by a warm but demanding British expat (the wonderful Jennifer Ehle). He must care for this single mom’s teen son, Trevor, who has muscular dystrophy and “realistically,” 7-10 years of life left. “So let’s try to do everything right.”

The patient, as such movies/novels demand, is a jerk. He insults, swears for shock value and fakes seizures just to rattle the new guy. He’s rigid about his routine, but he lets down his guard. There’s this map he’s marked up, every odd roadside attraction within reach, all pointing to “the World’s Deepest Pit” mine, a vast quarry that becomes their quarry when Ben, inevitably, suggests they go there.

Baby-faced Selena Gomez puffs on teen-friendly cigarettes and throws a little bluntness and flirtation Trevor’s way as a hitchhiker they pick up. Straining to ensure a more grownup film career after her child stardom, she relishes every profanity Burnett gives her. They’re like two TV-made conspirators who decided swearing is the difference between television and indie film.

No, it’s the diff between broadcast TV and premium cable TV — lame premium cable.

The backstories here have no real surprises — Ben is fleeing from a divorce papers process server and a tragedy in his past, Trevor has another important stop to make on the road and Dot (Gomez) is thumbing to Denver to get away from…something.

Rudd could play this guy’s light side and sad baggage in his sleep. And Roberts (“Submarine”, “22 Jump Street”) is adequate, if not memorable as this cardboard cutout of “handicapped.”

A couple of laughs are all we get, a touching twinge here and there. You can’t help but wonder if there was more in this material that Burnett lost track of. Like a lazy stand-up comic, he relies on F-bombs for laughs, without realizing how quickly they lose their comic effect with overuse.

MPAA Rating: TV-MA, profanity, sexual and scatological humor

Cast: Paul Rudd, Craig Roberts, Selena Gomez, Jennifer Ehle, Bobby Cannavale
Credits: Written and directed by Rob Burnett, based on a Jonathan Levison novel. A Netflix release.

Running time: 1:49