Do you know how to pronounce the English word "net"? Or "bet"? Because that's the vowel sound needed for a short "ä". Now, a short "e" always sounds exactly like a short "ä", so "merken" is pronounced /märken/, "wenn" is /wänn/ and "fertig" /färtig/ The other sound you'll need is the long "e". If you don't know how to pronounce that, I can only suggest trying to produce a long "ä" and then start changing it towards an "i" - it'll be sort of in the middle between the two. It's an important sound, because if a German word contains a long "ä" or "e", then it's considered correct if you pronounce both like a long "e", since lots of German native speakers pronunce "Mädchen" as /meedchen/ and "Käse" as /keese/ If this is too complicated, write a private message to me and we'll practice your pronounciation over skype. :)