Spanish Grammar/Definite and Indefinite Articles
Definite articles are words that translate to " the". Indefinite articles correlate to the English "a" or "an".
Spanish articles change form to agree with the noun they're modifying in number and gender. Therefore, there is more than one word for "the" and more than one word for "a/an":
The three elements at play here are definite vs. indefinite, masculine vs. feminine, and singular vs. plural.
The difference between definite and indefinite articles is the same as in English: you are either talking about something individual and specific (definite, "the - el/la"), or generally referring to something in a slightly abstract way (indefinite, "a/an - un/una/unos/unas").
Whether you use the masculine or feminine article depends on the gender of the following noun: abeja (bee) is a feminine noun, so 'the bee' is 'la abeja'. Bolígrafo (pen) is a masculine noun, so 'the pen' is 'el bolígrafo'.
Plurality is simple: if you're talking about more than one (or zero, same as in English), you use the plural form:
dos abejas - two bees
cero bolígrafos - zero pens