Why the letter "د" sounds sometimes emphatic?


Why is this letter sometimes emphatic like in the word house دار ?
Pourquoi cette lettre est parfois emphatique comme dans le mot maison ?

And how to know when the “د” is emphatic or not ?
Et comment sait-on quand la lettre “د” est emphatique ou non ?

Thank you.

Don’t hesitate to also correct my english lol, you can answer in french.