There are a few words that I have seen commonly confused, substituted for one another, and misunderstood, so I've made a list of the ones I hear the most.
Opala (oh-paw-law) and Mahalo (maw-haw-low)
These are only confused because they are both commonly seen on trash cans. Opala is the word that actually means trash; the mahalo is just posted thanking people for disposing of trash properly.
Confusing thank you with trash is something to definitely avoid!
Luau (loo-ow) and lau lau (lao-lao) and lua (loo-ah)
These are all very different words. A luau is a gathering where food is served for a special
occaision. Lau lau is a food made up of taro leaves and some type of meat (lau lau is eaten at luau, so the confusion is frequent). A lua is a bathroom.
kahuna (ka-hoo-na) and kupuna (koo-poo-na)
A kahuna is a priest or a skilled practitioner. A kupuna is an elderly person. Though they may be connected in many cases, there are also young kahuna, so it is important to know the difference.
Mauka means towards the mountains, or uphill. Locals say "up mauka" to indicate areas that are well inlaid and at high altitudes.
Makai means toward the ocean, or downhill. Makai is used more as an adjective, like "take the makai turn" or "on the makai side of the road".
A traditional district or land division that runs from the mountaintop to the shoreline. When looking on a map or asking directions, make sure to know whether you need to go the the mauka or makai part of a district.
The Kapu system was a set of political/thelogical laws deriving from Ancient Hawaiian society. Nowadays, it is used to indicate a restriction in entry to an area.
Holoholo means on the move (traveling). Locals use this to describe being out and about on pleasure trips.
I've repeated this word in this section because it is commonly used in lieu of an actual place. For example, locals will say "I'm just going holoholo" to indicate no real planned destination; or, to say they are in transit to a specific place.
A lua is a bathroom specifically with a toilet. This may refer to a building or a portable toilet, so its better to ask specifically for a structure with a restroom if that's what you're looking for.
A luau is a party with food, traditionally to commemorate something. A baby's first birthday is always cause for a luau, as well as weddings, anniversarys, funerals, and visitors. Large whole-village luau were held to commemorate the changing of peace/wartime.
A house or small building. In Kailua-Kona, a popular rental building is called Hale Halawai, which means "the house for gatherings".
A He'iau is a temple or shrine. One of the most well known is the Ahuena He'iau, the temple that Kamehameha himself spent time in on the grounds of the King Kamehameha Hotel.
This word can be traced back to the arrival of Captain James Cook. When the Hawaiians glimpsed the caucasians, they were believed to be deceased bodies because their skin was so pale. The Hawaiians thought that they were "ha 'ole), which translates to "with life breath".
Haole has expanded to include any foreigner. Although used casually, like in the name of the "Haole Surf" company, haole can be very derogatory if used inappropriately. It is often used by locals to negatively describe someone, like "they're such a haole" or "that was so haole".
A kahuna is a priest or skilled practitioner. In Ancient Hawaiian society, only people high up in the Caste system could become Kahuna, so it was a high position in society. Priests, canoe builders, bird catchers, and navigators could all be considered kahuna.
Kupuna are the elders in Hawaiian culture. This refers to all elders in your community, not just directly related ones. To be specific, you must say "my kupuna". Locals say "papa" and "tutu" for grandfather and grandmother.
The Ali'i were the Hawaiian royals. Since Hawai'i had no monarchy before Kamehameha the First, there is no real specific word for King, queen, prince or princess. Instead, adjectives were added to the word ali'i to make the designation.
This refers to people who are native to the Islands. Many businesses offer "Kama'aina" rates. This refers to a special rate or discount available to local residents. In many cases, you must show some proff of residence or ID to get this discount.
Keiki refers to a child or multiple children, it can be used in singular or plural "she is my keiki" or "they are my keiki".