- Vowel followed by 1 consonant, which is followed by a vowel = long sound, e.g. first vowel in: meter, geven, etc.
- Vowel followed by 2 or more consonants = short sound, e.g. first vowel in katten, honden, etc.
- Vowel followed by 1 consonant at the end of a word = short sound, e.g. the second vowel in all the examples above.
- Vowel at the end of a word = long sound, e.g. ga, zo etc.
- Two consecutive vowels = long sound, e.g. gaan, twee, beer
Short, as said, the rules above apply in general and as always with languages there are exceptions.
If you really want to be sure you want to divide the word into syllables: http://www.dutchgrammar.com/en/?n=SpellingAndPronunciation.05 . However, it would be silly to expect someone just learning a new language to be able to do that.
In the case of kinderen
- Kin - der - en
If it were to produce a long sound.
- me - ter
Not really. When I was taught Dutch (in the Netherlands at university, mind you), you only need to add the accents when there is ambiguity in the sentence. Accents on words that do not normally have them in Dutch, usually add emphasis.
For example: ik heb een boek vs. ik heb één boek.
Yes and as the first word of the sentence already has an accent it is not needed here. And certainly not on the first Capital letter. Try to google that and also read http://taaladvies.net/taal/advies/vraag/238/en_een_van_de/ É is not used in the dutch language
It will depend on what kind of device you use to access the lesson. When I view duolingo on a pc web browser it usually has the accented characters in little buttons below the box where you type. You could also use alt codes (good to learn how, if you don't know, it can be useful). On Apple, I haven't used one for a very long time but I think é is apple+e. On my phone it's the easiest; I tap and hold/long tap the letter and it shows me options for various accents.