"It is not for a long period."
Translation:Het is niet voor een lange periode.
Because geen generally means none, although used quite differently than in English, as you have seen. I imagine you were thinking something along the lines of, "Het is voor geen lange periode."? That would sound a bit dialectic...if that's a word, but it would get the idea across. However, I think they are looking for two things here: One, the translation of 'It is not for....' and two, your ability to work out whether or not you need to add an -e to the adjective. 'It is not for' would be most often translated as 'Het is niet voor', but when I think of the 'geen' sentence, I envision an old man, long clay pipe in his hand, wooden clogs, windmill in the background, tulips blowing in the wind as the sound of a canal lapping at its shores can be faintly heard. He's about to tell a story from his childhood and out comes that sentence. Whatever...I guess what I'm saying is, it isn't wrong, but it isn't what they want.
I can't say something, and sorry if I have a mistake, something like “Het duurt niet een lange periode”?
That's what I wrote and Duo says it's wrong :(
I wrote "Het duurt niet te lang" - I guess that literally translates to something else, but isn't language about conveying meaning?
It's such an odd sentence anyway, I wonder if it can be removed or further examples in every-day conversation can be given to strengthen the need to know this
I agree it is an odd sentence. You are right in that translating is about conveying meaning, but you also have to convey the right (correct) meaning. You don't want to say, "I think the enemy has put a mine on one of these roads" when you should be saying, "I think the enemy has put a mine on this road". You chose to say, "It doesn't last very long", whereas the Dutch sentence means "It isn't (or hasn't been) for quite a while". In English, it would be better to add '...of time' in my opinion, just to clarify things, but I still think the original Duolingo sentence is kind of dangling out there with nothing to hang on to. It might have been better to ask us to translate something like, 'He is away for a long period' which is still a bit odd but what can you do when we haven't gotten beyond present tense?
I keep getting lang mixed up with lange. And welk mixed up with welke. I don't really get it.
This is probably the hardest thing to deal with in Dutch, because it has to do with memorising the 'het' vs 'de' words (neuter vs common). But it is more than that, because, not only do you have to know this, you also have to know when it matters. Basically, usually, you will always add the -e to the adjective if it is a 'de' word OR if the 'het' word is preceded with 'the' something something, rather than 'a' something something. So, for example, if you say, 'Mijn hond heeft een lange staart." you add the -e because staart (tail) is a 'de' word. If you said "Mijn hond heeft een lang gezicht" you don't add the -e because gezicht (face) is a 'het word. This is the only time you will not add the -e, when you use 'een' (a). If you said 'het' (the): "Mijn hond heeft het lange gezicht" (yes, a strange sentence), then you add the -e to lang. With 'de' words, you always add the -e, so if you say, "Mijn hond heeft de lange staart" it is the same as the first sentence. In English these sentences are: "My dog has a long tail" "My dog has a long face" "My dog has the long face" and "My dog has the long tail". So the best way to remember when you add the -e is to remember when you DON'T, and that is when you use 'een' with 'het' words. The rule is more complicated, as you will learn later on, but for now, just stick with this and remember that every rule has exceptions so don't hunt me down and pour that beer all over me if you find one. Also, I'm Belgian, not Dutch, so what I know about the language sometimes differs from the Nederlanders, although in general, we follow the same rules. Now, for the sentence given here, you might say, well, it is a 'het' word, whatever it is, but in this case, 'het' is being used as the word "it", which 'de' never does. The adjective 'lang' describes the 'de' noun, 'periode', which just happens to end in an -e, a fairly uncommon occurrence in fact. One might expect the word to be spelled 'periood', but it is not. I hope this helps without too much confusion.
Not sure if it helps but Welsh has the idea of a definite or indefinite sense. Importantly (and why I raise it here) is that it is broader than whether some definite article is or isn't used. For some words this governs whether the first letter of an adjective changes (nightmare using a dictionary, as vach = bach so you find it under b but not under v!) So even if a word with the sense of "the" wasn't used, but the sense is definite, so for example "my" or "his" or "this" or "that". So what has this to do with Dutch? I had clearly quite wrongly seen "een" and misapplied the wrong rule. I should just concentratie on the Dutch, but am still unsure if I can get a rule from what has been posted.