There's tenses in Indonesian. But they are expressed with tense-particles (akan, telah, sedang, sudah, tengah, etc...), or context word (tomorrow, yesterday), and not by the conjugation, as there's no conjugations in Indonesian (but not no tense expression).
For instance. Saya makan. I eat. Saya akan makan. I will eat. Saya telah makan. I was eating/I ate.
So, it's the proof, there are tenses.
No conjugation is not no tenses, but as it's the case with the plurals, the tense can be implicit or explicit.
That's true that you can have a sentence with "Saya makan jeruk", and in some contexts, because you have words that express that it's a past action or a future action, in the previous sentence or the next sentence for instance, and it changes the tense of the sentence.
For instance, it's possible to say in Indonesian:
I was visiting my mother yesterday. Saya makan jeruk.
In this case, the past is carried by the first sentence "I was visiting my mother yesterday", and you don't need to indicate it in the second sentence with "telah".
It doesn't mean that you have no tense indicators, but the tense indicators are wider than in English (English has only conjugations as tense indicators), and can be in other sentences preceding or following the sentence (= context).
Tip: In Duolingo exercises, each time you have to tense-word indicators, like akan, telah, sudah, kemarin, etc... you have to use a present.
No, this is not the correct answer, because it's ungrammatical in English.
And you've understood a part about Indonesian plural but not everything.
There are 2 ways to express Indonesian plural. By the explicit form (jeruk-jeruk = oranges = explicit), and by implicit form, from the context.
So, if I am in a context where I know there are several oranges, I can say simply "Saya makan jeruk", and it means a plural.
That's a very important thing to understand, that there's ambiguity about singular/plural in Indonesian, unless you have the explicit singular (for instance, sebuah jeruk = an orange) or the explicit plural (jeruk-jeruk).
Thus, for instance, the sentence "I eat 2 oranges" can be expressed with either "Saya makan 2 jeruk-jeruk" or with "Saya makan 2 jeruk".
The second is even better, as it's not useful to express 2 times it's a plural, the "2" is enough in Indonesian to make us know.
In English, you would need an article with a singular, to make the precision if you want to talk about an orange or the orange.
Only plural can skip the article, for instance, when it's a generality:
Horses always eat grass.
I eat oranges = a generality, or undefined oranges.
I eat the oranges = definite orange, like the oranges that are on the table.
I eat orange, without an article is grammatically wrong in English.
"Sebuah jeruk" is an explicit singular. Sometimes it's needed, when you want the person to understand you talk about a singular, not a plural, as the plural-singular thing is ambiguous in Indonesian.
- Jeruk can mean the singular, or the plural (ambiguous)
- Jeruk-jeruk is an explicit plural, and can mean only a plural.
- Sebuah jeruk is an explicit singular, and can mean only a singular.
They didn't give "Saya makan sebuah jeruk" as the meaning is different.
In their sentence "Saya makan jeruk" it's ambiguous, it can mean I eat oranges, I eat the oranges, I eat the orange, I eat an orange.
But "Saya makan sebuah jeruk" is non-ambiguous, and only mean "I eat an orange" (explicit singular).
Yeah, "Saya makan apel" could be either "I eat a apple" or "I eat apples".
Word "sebuah" can be replaced by "satu buah" (or just "satu") because "sebuah" are combination of two word, "satu = one/a" and "buah = fruit".
The prep. "se-" can be combined to anything like, "semenit = one minute", "setahun = a year", "sejam = an hour", etc.
Also, even in the plural ones (to replace "satu") like, "seribu mil = one thousand miles", "seratus tahun = a hundred years" and also for "sepuluh bulan = ten months". And then, "satu ribu", "satu ratus", "satu puluh" never been use anymore on this day.
For millions, you can choose between "se-" and "satu" like, "sejuta dolar/satu juta dolar = a millions dollars". It has 50:50 probability when you meet the natives and they can still understand.
Except for billions, trillions, and so on, Indonesians still use "satu" rather than "se-" like, "satu miliar = one billions", "satu triliun = a trillions", and so on.