"The dogs drink water."
Translation:Los perros beben agua.
la/las/el/los are definite articles (translating to "the" in English), with the first two (la/las) being feminine singular/plural respectively and the latter two being masculine singular/plural. un/una are the masculine/feminine forms of the indefinite article (translated "a" in English). Whether the article is definite or indefinite, it must agree with the noun it modifies in number (singular or plural) and gender (masculine or feminine).