"The orange is sour."
Translation:De sinaasappel is zuur.
After any form of the verb to be when it refers to the subject instead of to a following noun, then the "predicate adjective" in Dutch or Nederlands is in its basic form zuur without the -e ending.
The common nouns are always preceded with the version of the adjective with the -e ending. "De zure man" or "een zure man"
Neutral nouns get -e ending only with the definite article. "Het zure meisje" but "een zuur meisje" and "geen zuur meisje"