Learn Icelandic: Ég hræðist hundinn.

The base verb hræða means to scare or to frighten. By adding the -st ending to make it a middle voice verb, the action reflects back onto the subject. So instead of I scare, ég hræðist means I become scared or I am afraid.

If I wanted to say I scare the dog, how would I say it?

You would take off the -st ending and use the base verb. Ég hræði hundinn means I scare the dog. The -st ending completely flips who is experiencing the fear!

Where is the word for of in this sentence?

You don't need one! In English, we use the adjective phrase afraid of, but in Icelandic, hræðast is a verb that behaves like the English word to fear. You just put the thing you are afraid of right after the verb, so it literally translates to I fear the dog.

Why is it hundinn instead of just hundur?

The word hundur is the dictionary form, meaning a dog. Because the dog is the object receiving the action of the verb, it changes to the accusative case (hund). Then, we add the definite article suffix -inn to mean the dog, giving us hundinn.