Hi Wei-Da. I'm not the best person to ask this question, I don't know much theory about sounds. Also how I hear Dutch vowels as a native speaker is different from how Dutch learners perceive it, making it difficult to say "this is how it sounds". However, I can advise you to just listen to Dutch native speakers, for instance: https://www.youtube.com/watch?v=I3cmrA7jM4E
This way you can slowly get used to the way things are pronounced in Dutch. Also watching the facial expressions of people while they speak Dutch can help (how far is the mouth open, what shape do the lips have). And then repeating out loud is a good way to get used to speaking yourself and improving your pronunciation. Keep in mind that the Duolingo voice regularly sounds a bit awkard, that's why I advise you also to listen to native speakers (there are lots of clips available for instance on youtube).
I'll help you out, Susande!
Thanks to many years of choral studies in other languages, one can become quite accustom to IPA pronunciations.
According to IPA rules, the length of the vowel between those two words are almost completely true. 'aa' gets the /a/ sound where as 'a' gets the /ɑ/ sound. /ɑ/ naturally will sound a little bit softer than /a/. Both are open vowels, but /a/ is found slightly forward and /ɑ/ is slightly in the back of the mouth. Take a look at the second link for IPA pronunciation. Hope this helps!
Source 1: https://nl.wiktionary.org/wiki/WikiWoordenboek:IPA#Klinkers (look at the second bullet point at the bottom of the page as well)