"The big animal? Yes, it is a big animal."
Translation:Het grote dier? Ja, het is een groot dier.
Singular masculine and feminine nouns take de, singular neuter nouns take het, all plural nouns take de. You can compare it to French le and la. See here for more detail.
See this explanation on when the additional -e needs to be added to adjectives. Example for a het woord (dier) and a de woord (vrouw):
- het grote dier
- een groot dier
- de grote dieren
- grote dieren
- de grote vrouw
- een grote vrouw
- de grote vrouwen
- grote vrouwen
Please see Susande's response above for specifics. "Het" in the question means "the" and "het" in the response means "it".
"Grote" is used in the question because it is a reference to a specific animal, "The big animal."
"Groot" is used in the response because the noun is a het-word AND is a reference to an unspecified animal, "A big animal."
So basically, the adjective will always be "grote", unless the noun is a het-word and is non-specific. This is one of the reasons why you need to know if a noun is a het-word or a de-word.
If you are a native English-speaker like myself, you just have to fight through this. It does get a little easier with repetition.