How does one know when to say "het" or "de"? Why is it de appel, but het brood?
Mostly through practice. There are some rules that you can look up, such as diminutives always being het-words ("de appel" - "het appeltje", "het brood" - "het broodje"), but aside from that, it's unfortunately all quite ambiguous and a fact of life when learning a lot of languages.
No. De-words are either masculine or feminine, and het-words are neuter. However, the difference between masculine and feminine words no longer 'lives' in colloquial Dutch as it does in, for example, German. That's why de-words are often said to be of the common gender.
This is because Dutch happened when english speakers met german speakers, and together they formed dutch, and took english's gender neutrality.
There are no rules for het and de. You just have to learn wich words are with de and wich words are with het
What is the difference between Het and De they both mean the and i dont know what the difference is