Esperanto is a constructed language based on European languages (mostly English and Romance languages with a few influences from other Germanic languages and Slavic languages). Some European languages have both definite (“the”) and indefinite (“a”, “an”) articles, while others have none at all (such as most Slavic languages, where “a car” and “the car” would be the same).
Esperanto falls somewhere between these two categories. There is a definite article, la, but no indefinite article (interestingly, among European languages, only Celtic languages, such as Irish and Welsh, have a separate word for the definite article but no word for the indefinite article).
When an Esperanto noun is preceded by la, you can be almost certain that the correct English translation uses “the”:
I see the cat.La hundo ŝatas min.
The dog likes me.
By the way, don’t let your knowledge of French or Spanish fool you—there is no grammatical gender in Esperanto. The article la is used for all nouns:
la viro – the man
Since there is no “a” in Esperanto, how do you express indefiniteness? You simply omit the definite article:
I see a cat.Hundo mordis ŝin.
A dog bit her.
It works exactly the same way in the plural:
I see the cats.La hundoj trinkas.
The dogs are drinking.
However, since there is no plural indefinite article in English, the structure in the plural is actually the same as in English:
I see cats.Hundoj ŝatas min.
Dogs like me.
Does that mean that “la” = “the” and “no la” = “a”? Not quite. There is a third type of article in English. Uncountable nouns (like “water” or “rice”) are often not preceded by an article when they refer to a substance in general. However, in Esperanto, there is no distinction between this so-called zero article and the indefinite article; they are both implied by the absence of “la”.
This makes Esperanto easier to use (it doesn’t matter whether a noun is countable or not, so you don’t run the risk of making mistakes like saying “an information”), but it also makes Esperanto a tiny bit harder to translate into English because you have to pay attention to the countability of the noun, e.g.
I like coffee.
Translating “mi ŝatas kafon” as “I like a coffee” would be a mistake. The sentence is about coffee in general, not about “a coffee”.