Breakdown of Náunginn í búðinni hjálpaði mér mikið.
ég
I
búðin
the shop
mikið
a lot
í
in / into / to (before the hour, in telling time) / per (in frequency expressions)
Questions & Answers about Náunginn í búðinni hjálpaði mér mikið.
Why is it í búðinni and not í búðina?
The preposition í takes the dative case when it describes a static location, meaning where someone or something already is. Since the guy is located in the shop, we use the dative form búðinni. If someone were walking into the shop (showing motion or direction), you would use the accusative form í búðina.
Why do we use mér instead of mig after the verb hjálpaði?
The verb hjálpa (to help) always requires its object to be in the dative case. Even though me is a direct object in English, in Icelandic you must use mér rather than the accusative mig. This is a specific rule for this verb that you just have to memorize.
How does the noun náunginn get its definite article?
The dictionary form is náungi (guy or fellow), which is a weak masculine noun that ends in a vowel. When you attach the masculine definite article -inn to say "the guy", the final -i of the noun drops off. This gives us náunginn instead of a double vowel.
Could I use mjög instead of mikið to mean "a lot" here?
No, mjög means "very" and is mostly used to modify adjectives and other adverbs (like saying "very good"). To describe how much an action was done (like helping "a lot"), you need to use the adverb mikið.