Collocations for frightening - adj.

be, feel, look, seem, sound
become, get
The situation was getting frightening.

make sth
What can we do to make the experience less frightening?

find sth
He found the responsibility rather frightening.

extremely, fairly, very, etc.
genuinely, positively, truly
almost
a little, slightly, etc.

for
This is extremely frightening for elderly people.

to
It was all very frightening to a small boy.

Frightening is used with these nouns:
prospect, scenario, thing