Learn Hardcore Esperanto: La infano estas vigla.

La is the definite article in Esperanto, equivalent to "the" in English. It comes before a noun (in this case, infano) to specify that we are talking about a particular child.

What role does estas play in this sentence?

Estas is the present tense form of the verb esti, meaning "to be". It functions as the linking verb that connects the subject (la infano) with the predicate adjective (vigla).

How is the adjective vigla formed, and why does it end with -a?

In Esperanto, all adjectives end in -a. The word vigla means "lively" or "energetic". The consistent -a ending helps signal to learners that it is an adjective. When describing a singular noun—as in this sentence—vigla is correctly used in its base form. For plural, the adjective would take an additional -j (e.g., viglaj) to agree in number with the noun.

Why is the sentence structured as La infano estas vigla rather than, for example, Vigla estas la infano?

The standard word order in Esperanto is quite similar to English: subject-verb-predicate. La infano is the subject, estas is the verb, and vigla is the predicate adjective. Although Esperanto allows for some flexibility because of its clear grammatical endings, sticking to this SVO (subject–verb–object/predicate) order typically makes sentences easier to understand.

Do adjectives in Esperanto agree with the nouns they describe regarding number or gender?

Yes, adjectives in Esperanto must reflect the number of the nouns they modify. In this sentence, vigla is used with a singular noun (infano), so it appears in its base form. If the noun were plural (e.g., la infanoj), the adjective would also take a plural ending, becoming viglaj. As for gender, Esperanto adjectives are inherently gender-neutral and do not change form based on gender.

How can comparative forms be expressed using vigla?

Comparative and superlative forms in Esperanto are created using the adverbs pli for "more" and plej for "most" before the adjective. Thus, "more lively" would be pli vigla and "most lively" would be plej vigla. This method applies broadly to all adjectives in Esperanto.