Learn Icelandic: Prófin stressa mig mikið.

Yes! Icelandic sometimes adopts foreign words by simply adding the infinitive ending -a. You will see this with other modern words like prenta (to print) or gúggla (to google). It acts as a completely normal, regular verb.

Why do we use the accusative mig instead of the dative mér here?

The verb stressa governs the accusative case. While many verbs related to personal feelings or experiences take the dative (like líka or finnast), verbs that describe a direct physical or emotional impact on someone often take the accusative, treating you as the direct object of the stress.

How can I tell that prófin is plural in this sentence?

The noun próf is a neuter word. For neuter nouns, the singular definite (the exam) is prófið, while the plural definite (the exams) is prófin. You can also double-check this by looking at the verb: stressa is the plural form. If it were just one exam, it would be prófið stressar.

Why use mikið for 'a lot' instead of mjög?

When you want to say an action happens 'a lot' or 'much', you use the adverb mikið to modify the verb (in this case, stressa). You would use mjög (very) if you were modifying an adjective or another adverb, such as saying you are mjög stressaður (very stressed).