What's the difference between "to frighten" and "to scare"? I've heard both, but have never been able to figure out the difference.
I would suggest that 'frighten' is more intense than 'scare'. Although they are (very) similar, being scared is less serious than being frightened. That is definitely a second-order effect though; to a first approximation, they are (almost) equivalent.
The two words are synonyms and may be used interchangeably. Scare comes the Old Norse word skirra meaning "frighten."
I would use frighten when I mean something with the hint of a surprise. Scare would mean something more gradual, as in "to scare with fear, uncertainty, and doubt (FUD)"
When I use frightened I use it like it has a bit of tense and suspicion in it. Scared just feels like it’s horrible and creepy. Also frightening is more severe than scared.
First of all, I agree with Jonathan Leffler — "frightening" is more intense than "scary," but many do not really make that distinction and use them quite interchangeably. Unlike using "can" when "may" should be used, most people will not correct another if they use "scary" when it is felt that "frightening" is a better choice of word. The difference between "scary" and "frightening" is not as well defined as "can" and "may." Furthermore, the difference between "scary" and "frightening" is not taught in school.
Even if you want to help a person out by suggesting a more accurate descriptor, it isn't likely to go over too well. For example, if you were to meet someone who had survived the Paradise wildfire and they described it as "scary" and you corrected them by saying, "Do you mean frightened?" you're going to come across as a real jerk.
Perhaps the best way to understand the difference between two words is to put them into context. I'll start with this:
Hearing ghosts rattle their chains in a Halloween haunted house is scary; hearing someone yell, "Heil Hitler, Heil Trump" in a theater is frightening.
I'll add to it if I think of others.