Woops, I was assuming kanske came from a Swedish form of chance, so the leap to perchance - perhaps - maybe wouldn't be far.
But it's derived from kunna [Old Norse: can, to be able / know how to do ] + ske [Old Swedish: happen ].
Still an effective mnemonic, though (in my head anyway) :D
I changed the text there into this, which I think is more accurate:
Unlike English, adverbs are always placed after the verb in sentences that start with the subject. This is because of the V2 rule – the verb must always come second.
It is definitely possible to put the adverb before the verb, but
1. then the subject must go after the verb
2. this is a less common word order
But you can definitely say things like Långsamt går jag hem 'Slowly I walk home' – it can easily sound poetic since it is a less common word order, but it's a totally correct sentence.
There's a longer post about word order here: https://www.duolingo.com/comment/8970470 that may also be helpful.
Yes, I think kanske is the only real exception, but the rule it can break is V2, there isn't a rule that says the adverb absolutely has to go after the verb in main clauses. (Adverbs that describe the verb action usually go after the verb in main clauses, but that's a tendency, not a rule).
I would say "Är de kanske bröder?" is a possible translation (as Arnauti explained above, "kanske" is a possible exception to the V2 rule), but it's not the most common one. It also gives me a different feeling than Duolingo's sentence; Duolingo's sentence makes me think of someone speculating that they may be brothers, while your question makes me think of someone asking someone else if they happen to be brothers, by any chance.
De and dem are most of the time pronounced "dom". There are certain situations where they can be pronounced "de" and (presumably) "dem" respectively, but you'll never go wrong pronouncing them "dom" always.
De lagar mat -- pronounced "dom"
De blåa skorna -- can be pronounced either "dom" or "de"
I'm not a Swede, so don't completely take my word for it. But this is just information I'm passing down here.
Too me, "Maybe they are brothers?" and "Are they maybe brothers?" Have different meanings to me. The former is rhetorical while the later, albeit funny sounding (thought I do hear it sometimes), is a non-rhetorical question. Do both of these meanings apply to this sentence, or would they latter be better translated as "Är de kanske bröder?"
The question mark is evidently correct in Swedish, but in English this sentence is a statement. One can make it a question with inflection when speaking, but it has to be written with a period, not a question mark. And without the inflection, in speaking it is also a statement. "Maybe they are brothers" does not require an answer.
The word order seems funny to me. I would have put the "kanske" at the beginning. I guess as an English speaker I'm doing a direct translation (which I know doesn't always work) and seeing this example as saying "They are maybe brothers." Of course, that sentence would be very weird in English.