It depends on two factors:
1) Whether it is the definite article or the indefinite article.
2) The gender of the word that follows.
Formally, there are three genders in Dutch: masculine, feminine and neutre but for this excercise you only need to know the difference between "de-words" (= masculine and feminine words) and "het-words" (neutre words).
With the definite article ("de" or "het"), there is always an ending "e" for the adjective.
De grote tafel ("de-word"), het grote glas ("het-word").
With the indefinite article ("een"), only "het-words" lose the ending "e" for the adjective.
Een grote tafel, een groot glas (de tafel, het glas)
There are (rare) exceptions: e.g. you might encounter "een grote man ("de-word") but also "een groot man".
"Een grote man" means that the man is tall. Een "groot man" has another, metaphorical meaning; it's a great person and it does not refer to his height. But these exceptions are rather rare.
