So, in English, it would not be unusual to say "Man always needs water." (At least, not unusual in a grammatical / stylistic sense.)
So I always try to translate this sentence that way, and always I am wrong.
Can someone explain to me why "Man always needs water" is an incorrect translation of this, and what the Swedish version of such a sentence would be?
That "man" should be människan in Swedish, but actually you could pretty much take your pick between the different number/definiteness combinations, I'd say all of Människan behöver … , En människa behöver … , and Människor behöver … are acceptable translations in this case (because definiteness doesn't work the same in Swedish & English here).