The answer? - You can't.
I know this is not the answer you were waiting for, but that's it. You can't know, there is no trick or giveaway in the vowels of the word (sceptics: keep reading!). There are a few rules if you Google them, but this comes with a warning: it will give you a huge headache and will not get you anywhere but in a very confused state of mind.
Is it annoying? Yes. Very annoying. Is it important? No!
As it is (almost) impossible to guess whether a word should come with DE or HET... This may cause a lot of frustration among students and will even drive some students towards giving up on learning Dutch. But in reality it is important to think about the goal you are trying to achieve. Is it important to use the correct article to get your point across? Will the person who is listening, stop understanding you when you pick the wrong one?
Dutch people in general are pretty amazed when people speak their language. When you try, they feel like you are taking a step towards them, they feel closer to you, they appreciate your effort. The article mistakes, they don't even notice, as you are focused on communication, on the connection you are making together.
So my advice is this: try to learn the article with each new word, but don't sweat the small stuff, allow mistakes to be made, and have fun! How? Well, I will teach you two tricks after all. I know I said there were no tricks, but here is two, that will make you have some fun in applying them.
If you want to use HET all the time, you just make all the words small. Het boekje, het woordje. Everything sounds so cute! If you prefer to use DE all the time, make everything plural. DE boeken, de woorden.
Works great, right? Not sure if you are getting your point across though... so shall we just go back to communicating and in an imperfect, but effective way?
Let me know how you feel about de and het, and how you deal with it, by leaving a comment below :)
TOT DE VOLGENDE KEER!