This is a copy-paste from another thread.
Whose cat is it="hvis kat er det". Is it/that my cat= "er det min kat" It is my cat="det er min kat". It is a cat= "det er en kat".
But why because "kat" is clearly an n-word, 'the cat="katten", and you have been taught to use n-words with n-words, and t-words with t-words. For ex. 'that car is mine'="den bil er min" or 'that watch is mine'="det ur er mit".
The way to think about it is this, in my 'cat' sentences above, the central subject was not the cat but 'it'-the unknown, and when that is the case 'it' always ,AFAIK, translates as "det". I say AFAIK because i cannot imagine a sentence where it would'nt.
Ex. you and a friend see something in the distance and have the following exchange. What is it="hvad er det. Is it a car="er det en bil. It looks like a car="det ligner en bil". It is definitely a car="det er helt sikkert en bil". That car is green="den bil er grøn". Even in the 'it is definitely a car' example 'it' still translates to "det" because the sentence is still about what exactly 'it' is.
There are cases where 'it' would translate as "den" but these are when what 'it' is, is not the central part of the sentence, but an n-word noun is. For ex. 'the film is good, have you seen it'="filmen er god, har du set den", or 'your car is beautiful but is it fast'="din bil er smuk men er den hurtig", or 'the food is as good as it is expensive'="maden er ligeså god som den er dyr".
De, dem and deres can have a different meaning in a very formal polite context. For instance if you were to speak with the queen. De = You, Deres = Your. example: "Vil de have deres mælk?" = Would you like your milk. As opposed to the common way of saying it: "Vil du have din mælk?"
Different vocal sounds. The "e" in "Det" is pronounced like the "e" in "estate" (depending on your pronunciation of course, but that vocal sound isn't really used in English, so it's the closest I can think of), while the "e" in "De" is pronounced like... well, the English letter "e" (or the Danish letter "i").