Learn Icelandic: Hundurinn hræðir mig.

This sentence uses the active base verb hræða (to scare), so the dog is doing the scaring to you. If we added the -st ending to make the middle voice verb hræðast (to fear or be afraid of), the sentence structure would flip. You would say Ég hræðist hundinn (I fear the dog).

Why do we use mig here instead of ég?

Mig is the accusative (object) form of ég (I). Since the dog is the subject performing the action, and you are the object receiving the scare, you must use the object pronoun mig.

How do we know it is 'the dog' and not just 'a dog'?

In Icelandic, the word for 'the' attaches to the end of the noun. The base word is hundur (a dog), and adding the definite article -inn turns it into hundurinn (the dog).

What is the dictionary form of hræðir?

The dictionary form is hræða (to scare). Since the subject is a third-person singular noun (the dog), the verb takes the ending -ir and changes to hræðir.