"The big boy. The boy is big."
Translation:De grote jongen. De jongen is groot.
It isn't plural and singular, it's where the adjective is - if it's in front of a noun with a definite article, an de-noun with an indefinite article, or a de-word otherwise, you need to add the e to it. In the second sentence, it's a predicate adjective, so it doesn't get the e. The lesson Adjective basics has the details and some examples on this.