You can see from the trailer that I Married a Witch is a wacky comedy that might remind you of the TV series Bewitched. In fact, the concept behind Bewitched came from this 1942 film, along with 1958's Bell, Book, and Candle.
But the concept underlying I Married a Witch is deeper and darker. The witch Jennifer (Veronica Lake) was burned at the stake in Salem, Massachusetts, in the 17th century. She cursed her accuser, Jonathan Wooley, and all his descendants. When her spirit is freed in 1942, she confronts one of those descendants, Wallace Wooley (Fredric March), determined to make his life miserable. She attempts to feed him a love potion, but the movie's twist is that Jennifer consumes the potion instead, and falls in love with Wallace.
We know that the witch trials in Salem and other places were a dark spot in history, during which inconvenient women -and some men- were tortured and killed unjustly for a number of reasons, such as revenge, fear, mob mania, deflection, or to gain relative piety (and no "witches" were burned at the stake in Salem- they were mostly hanged). In 1942, the evils of a bygone era were valid fodder for a comedy of errors.
That said, the movie is quite funny. You can now watch the full-length movie I Married a Witch. Keep an eye out for elements that were later incorporated into Bewitched.