Translation:They ordered coffee, however they did not order food.
Namun is usually translated as 'however', as it's more formal than 'tapi' for 'but'.
To my understanding, there is no tense in Bahasa. The time which a sentence is referring depends only on the conversation context. Using pass tense for the sentence seems more logical in general.
Although you can argue this is a conversation between waiters, in which case, describing a gang of usual customers that always only order coffee in the waiters' restaurant.
In English we mark tense on our verbs, but Indonesian doesn't. It's all understood through context. But if you want to make the time explicit, you can use adverbs. For example, "sudah" indicates that a verb was completed in the past, and "akan" indicates that the verb will take place in the future.
So, for this sentence ("Mereka pesan kopi, namun mereka tidak pesan makanan"), past tense is just one option. It can also be translated as "they are ordering coffee, but they aren't ordering food."
Hope that helped.
It should do. I must admit to being a little frustrated to feel this course is judging my English (I'm a native English speaker) when what I'm actually trying to do is get into the mind and way of speaking of an Indonesian!
In English one could simply say " They ordered coffee, but not food" and this would be a valid, and perhaps a better, translation of the Indonesian sentence.
I think that if the tense it to change between clauses in the one sentence, then temporal markers should be used.