Difference between De and Het
Is it random or is there a rule to it. Please help me.
'De' is used for nouns that are masculine or feminine, 'het' is used for gender-neutral words.
For example, 'het gebouw' (the building), 'de auto' (the car), 'de vrouw' (the woman), 'de man' (the man). This is something I think you should study by heart for each word, I don't think there are any tricks for it. Usually you can assume that you must use 'de' when the noun is a person (like "man" or "vrouw"), but that doesn't always count cause for example "kind" is gender neutral so you must say "het kind".
Also adding a diminutive ending always makes the word neuter. So it could be "de man", but it's also "het mannetje". It does come from PIE, so there is to some extent some rhyme and reason to it if you know anything about how the Germanic languages worked originally.
For example, -ing words are always feminine, as they are in all other Germanic languages. Diminutive words are always neuter, again, as they are in all other Germanic languages.
On the whole though, it's just something that has to be memorized. You can't logic it out in terms of association, use, or ontological category; there is a way to logic it out for the most part, but that would require you to learn some Proto-Indo-European and Proto-Germanic roots, and at that point you might as well just memorize the Dutch words.
plurals get the "de" article. Het plane, De planes, Het building, De buildings. As for man, woman, boy they are "De", girl is an exception it is "het". But as Izim and Ofred are saying you pretty much have to memorize by word there does not seem to be a clear cut rule that makes it easy.
As the other commenters have said, essentially you have to learn as you go whether a noun is a ‘de’ word or a ‘het’ word. But there are ways to give your memory a helping hand until the forms become fixed in your mind.
It seems to be generally accepted that visual images can assist people when they’re learning a new language. What I specifically do in this particular case is imagine two characters from a TV programme and assign ‘de’ words to one of them and ‘het’ words to the other. I use Chidi and Janet from The Good Place - I give the ‘de’ words to Chidi and the ‘het’ words to Janet and they act them out for me in brief scenes. Sometimes they’re acting words out on their own or sometimes they’re in the scene together. For instance, Janet owns ‘het restaurant’ and offers ‘het menu’ to Chidi. Then she brings ‘het eten’, which becomes ‘de maaltijd’ for Chidi. That sort of thing. And I try to make the scenes silly and entertaining - make them as memorable as possible.
I realise people learn in different ways so this might not be your sort of thing. But I thought I’d mention it in case it helped, and it might give you a starting point to find your own memory techniques.