This is the 482nd most frequent Estonian word.
ainus
Only; sole; single.
Here the word "ainus" is used as an attributive adjective, and it specifies that there is only one person capable of answering the question, emphasizing exclusivity.
Tema on ainus inimene, kes sellele küsimusele vastata oskab.
He is the only person who knows how to answer that question.
In this sentence, "ainus" functions as a determiner, indicating that the opportunity is singular and emphasizing its uniqueness.
See on ainus võimalus, mida meil praegu on.
This is the only opportunity we have right now.
Here, "ainus" is combined with "ainult" for emphasis, serving as a predicate adjective to stress the exclusivity of the solution.