"She loves surfing."
Translation:Puni ʻo ia i ka heʻe nalu.
You would not be able to use the verb phrase Ke he‘enalu nei after an object marker like i. So the gerund would be either just the noun or you can put ‘ana after it - i ka he‘enalu or i ka he‘enalu ‘ana. Really in this case, only i ka he‘enalu works because it is used as a noun/activity.
What's the meaning of "Puni i ka he'e nalu 'o ia" then? I just don't understand when the structure of sentences. Sometimes Puni(the verb) comes first (like in this example), sometimes it comes after (like "He tūtū kāne puni ʻawa ʻo ia."). Sometimes the subject comes at the end (like the previous example), sometimes like in here not. I'd be very thankful if someone could give me a short explanation...I'm so confused...everytime I think I understand, there's a version of a sentence I fail again.
No one would say it as "Puni i ka he'e nalu 'o ia" . You could say "Puni he'e nalu 'o ia, though.
You have to open your mind up to a very different grammar structure that you are dealing with. You correctly identified that Puni starting a sentence would generally be as a verb, but in that second sentence, puni is used as a modifier / adjective. In Hawaiian, words can be used in many ways, noun or verb or adjective or adverb.
"He tūtū kāne puni ʻawa ʻo ia." - puni ʻawa describes the tūtū kāne - He is an awa loving grandpa.
Let's look at some examples, first with a verb. ʻŌlelo ‘o ia i ka ʻōlelo Hawai‘i. --> He/She speaks the Hawaiian language.
That direct object ʻōlelo Hawai‘i can be put right after the verb kind of like an adverb - ʻŌlelo Hawai‘i ‘o ia.
Haku ‘o ia i ka lei. --> Haku lei ‘o ia.
Then there are sentences with no action verb.
He tūtū kāne ‘o ia. --> He is a grandfather.
He tūtū kāne puni ‘awa ‘o ia. --> He is a grandfather who loves ‘awa, or as they are saying it, He is an ‘awa loving grandfather.