The article 'a' or 'the' has to be understood form context.
Verbs are usually given in their imperative singular form in the dictionary. They form their three main tenses from three different bases. Often the bases are identical. But in most other cases they are different.
The verb iru 'to be' is in its imperative singular form. Its present tense base is 'iruk'. In all three tense forms, verbs have different endings for different persons in the singular and plural numbers.
The 3 rd person singular neuter form of iru is iruku.