"Mathematics is my friend."
Translation:Matematika adalah teman saya.
Hai! The use of "adalah" would be optional here, of course. But I think this applies better to the colloquial speech. Also, a context would make necessary the difference (to avoid misunderstanding) if "matematika" is about maths skills. For example:
Matematika saya / My maths (skills)
Matematika saya bagus. / My maths is excellent.
Matematika teman saya / My friend's maths (skills)
Matematika teman saya jelek. / My friend's maths is bad.
In contrast to the Duolingo sentence:
Matematika adalah teman saya. / Maths is my friend.
I hope not to use bad English here, but what I mean is that the context can change the meaning of Indonesian words a lot.
Selamat belajar! :)
So, the "adalah" is needed to clarify, because the sense of the sentence is rather exotic?
Incidentally, your English looks great to me, better than my writing in any language other than English. The one somewhat exotic element is your greeting, "hai," which would probably be either "hi" or "hey," at least in American English.
Yes, I think there are sentences that need to remark the subject, and in this case, the stress would be on "matematika". I have read that Indonesians can also use gestures to indicate the stress in a part of the sentence, then the subject is comprehensible without "adalah". But I would like to know more about colloquial speech, for example if Indonesian can use a kind of stop between the words that facilitates the comprehension.
Thanks again for your kind words about my English. I always appreciate the sentence comments because they help me to improve both languages at the time. And I am using "Hai" as an Indonesian greeting, and I would prefer to use "Halo", instead. I always prefer to use "Halo" in online conversations. And "Hai" sounds just the same as the English "Hi". Indonesians indeed can write and say "hai" and "hey". I think I will try to use "Halo teman" (as in the Swahili course I use to say "Mambo rafiki") or "Halo saudara", "Halo saudari" (I think people in Jakarta have these kinds of greetings, but I am not sure if they can mean "Hello brother", "Hello sister", or just "Hello Sir / Mister", "Hello Miss / Mrs"), also I know there are many kinds of addressing. I feel I would prefer more "Halo teman" or "Hai teman", even if it could not be appropriate in formal situations. Some people from the embassy in my country has always received me with a smile when I speak to them with "teman", "teman saya", "guru" (teacher, in the class of gamelan music), and perhaps this is because they also know about Rioplatense Spanish, "mi amigo", "eh amigo", "maestro", "mina" (girl, woman, not little girl), "doña", "don", "capo" (boss, a wise person, also used between friends, workers), "mi viejo" (my father), "mi vieja" (my mother), and many other expressions. Another Indonesian word I really like is "Mas", for masculine addressing, and the feminine would be "Mbak", but perhaps they are not Standard Indonesian. For example, to express my gratitude for your comment here, I would say: Terima kasih Mas James untuk komentarmu / komentarnya. (I think this is polite, and using Anda is just very formal; "komentarmu", "komentar kamu", meaning "your comments", or "komentarnya", just "the comments)
Semoga sukses dan selamat belajar! :)