These words are often confused. Although the words afraid, scared and frightened have similar meanings, the grammar is not the same. These are all adjectives and express nearly the same degree of fear. In many cases, they are interchangeable.
Frightened is mainly used to talk about a sudden fear.
Many children are afraid of / scared of / frightened of cockroaches and spiders.
Don’t be scared / afraid / frightened. He is a kind person. He won’t hurt you.
Note that all of these three words can be followed by of + -ing form.
He never drives fast. He is scared of / afraid of / frightened of crashing.
I didn’t go hiking. I was afraid of / scared of / frightened of injuring my knees.
Frightened is not normally followed by of + pronoun.
He is a stern master. The boys are all afraid of / scared of him. (More natural than ‘The boys are all frightened of him.’)
All three of these adjectives can be followed by a to-infinitive.
She was too scared to raise her voice.
I am not afraid to tell the truth.
I am too frightened to stay alone in this house.
Things can frighten or scare us. Or we can be frightened by or scared by something. We cannot be afraid by something.
She was frightened by that haunting tune.
They were scared by the explosion. (BUT NOT They were afraid by the explosion.)
Afraid is not normally used before a noun. It goes after the verb.
She is afraid. (BUT NOT She is an afraid woman.)
Scared and frightened can be used in both attributive (before the noun) and predicative (after the verb) positions.
She is a frightened woman. OR She is frightened.
Frightened / Frightening
As a general rule, adjectives ending in –ed (e.g. frightened, interested, bored, excited etc.) are used to say how people feel. Adjectives ending in –ing (e.g. frightening, interesting, boring, exciting etc.) are used to describe the things or situations that invoke those feelings. Frightening experiences make people frightened. In the same way, terrifying experiences make people terrified.
It was a frightening experience. (= That experience frightened me.)
I was frightened by the gunshots.
She looked very frightened when I told her that she would need to be operated on.
Terrified / terrifying
In the same way, you can use terrified to describe how you feel and terrifying to describe the thing that terrifies you. Note that terrified and terrifying are used to talk about a greater degree of fear or anxiety.
It was a terrifying ordeal. I am glad that it is over.
I was terrified when I heard the explosion.
Scared / scary
Scared is similar to terrified and frightened. It shows how people feel. Scary is similar to frightening.
I felt scared when the lights suddenly went out.
Spending the night all alone in that big bungalow was scary.
Scared and frightened have very similar meanings and indicate fear or anxiety of the same degree.