"I have an apple."
Translation:Saya punya apel.
They're the same word (base word is "punya"). I'm not certain about this next part, but I believe that "mempunyai" is the transitive form of "punya". That means that it takes an object, whereas the regular "punya" is intransitive, and doesn't necessarily take an object. Of course, this is all wading into Indonesian grammar. I've never heard "mempunyai" in everyday Indonesian. It's reserved primarily for legal documents and formal instances.
"Punya" is the base word of "mempunyai". When you turn "punya" into verb form you add the "mem-" and "-i" to the word. When changing a base word into a noun, verb or adjective in Indonesian there are usually certain parts added onto the word and this also depends on the first letter of the base word. If you have a base word that starts with "s" when you transform that word in to me- verb form you replace the "s" with "meny-". For example: The word "Siap" means something along the lines of "ready" so when you put "Siap" into verb form it becomes "Menyiapkan" which means to prepare this is because you take away the first letter of the base word and add the prefix and suffix which if the word starts with S it will be "meny-" and sometimes "-kan" on the end. Im sorry if this isnt easy to understand because I am not great with writing in english (even though its my native language)
Here are some "me-" prefixes which are added onto base words to become verbs: "Me-", "Mem-" (e.g. "Membaca" or "Memakai"), "meng-" and "Meny". There is also more types of prefixes that change the base word which I can also explain if you like. Like "Pe-" and "Pe- ... -an" for making the word into a noun.
Oh, it is a part of any language that I hate the most — collective nouns. Collective nouns are called "penjodoh bilangan" in Malay language; not sure if it's the same for Indonesian. "Se-" is a prefix to mean "a/one" and "buah" literally means "fruit." So, "sebuah apel" just means "a fruit of apple. Others include "seberkas kunci" for "a bunch of keys, " or "sebotol susu" for "a bottle of milk." It is more literal here than in English, which is good, and you rarely encounter situations like "a murder of crows" here in Malay and Indonesian.
"Saya ada sebuah apel" is correct, but uncommon in daily uses. Besides, if we learn from basic, the word "have" literally means "punya" in indonesian. "Saya mempunyai sebuah apel" is correct and complete sentence, but more formal than "Saya punya apel". Next, "my apple" is translated as "apel saya", which maybe has similiar mean, but not exactly the same. Yes, Indonesian and Malay are slightly different in terms of grammar and some common words