It's a little difficult to hear the difference between "De man" (my answer) and "De mannen". Perhaps I need to use earphones (or perhaps I'm getting old!). ;)
Sorry, Handrisuelo, I'm afraid that, actually, you are the one who's wrong.
I've just listened to it and it clearly says /ˈmɑnə(n)/.
What's your mothertongue? Perhaps in your native language there are no schwas (/ə/), and that's why you're having difficulties hearing it.
As a rule of thumb you add -en to words with one syllable, de man > de mannen while you add -s to words with more than one syllable, het meisje > de meisjes There-s also a handful of nouns that take irregular plurals such as de stad > de steden. But those ones oughta be learnt by heart.
it hurts me to write stuff like foto's, i'll admit. if it helps, 's for plurals is only used when the noun ends with a short vowel. the only exception is a noun ending in e (e.g. ziekte). these nouns have no apostrophe.
this site explains it more: http://www.dutchgrammar.com/en/?n=NounsAndArticles.11 this site has been really helpful to me for multiple aspects of grammar (such as conjunctions, as which ones are coordinating and which are subordinating is pretty different than in german (i've studied german for years which is why i mentioned it at all).
When you got 'man', you see a short vowel. In the plural it should remain like this, a short vowel. To that purpose, you double the 'n', because a vowel is short when it's followed by two consonants. Then you add the -en. That's why it's mannen, or that's what I read. The word 'maan' has a long vowel, so it should remain like this in plural and the plural will be 'manen'. Then, when a word finishes with a vowel (and many foreign words), you add 's like 'foto'. Then some words add -s because they had some endings like -sje, 'meisjes'. And others are irregular...
That's because there's a change in word category: en is a coordinating conjunction, while een is an indefinite article.
On the other hand, skipping a letter in mannen doesn't necessarily produce a change in category, since manen is a noun meaning mane. It can also be a verb, though, meaning admonish, but I'd say those are low frequency words, so it's mainly interpreted as a misspelled form of mannen.
Being so used to learning Norwegian, Dutch can be confusing and a little bit frustrating for me:
the masculine indefinite article (a/an) in Norwegian = 'en', but in Dutch the indefinite article is 'een' (you can imagine the endless amount of times I have written and will write 'en' in place of 'een' before either not noticing and getting it wrong, or noticing before I press check, sighing at myself and having to go back and correct myself)
'mannen' in Norwegian translates to 'the man', but in Dutch it's 'men'. AHH! I'm so used to Norwegian I sometimes forget other languages don't use suffixes to make an indefinite into a definite.
'de' in Norwegian = 'they', 'de' in Dutch = 'the', not to mention the struggle not to pronounce 'de' as dee as it is in Norwegian.