единствено

In a sentence

• Трябва да постигнем единствено най-доброто.

  We must achieve only the best.

  Here 'единствено' is used adverbially, emphasizing exclusivity.

• Това е единственият възможен избор.

  This is the only possible choice.

  Here 'единствено' serves as an adjective, describing the singularity of the choice.

• Единствено Иван разбира това правило.

  Only Ivan understands this rule.

  Here 'единствено' functions adverbially, singling out Ivan as the one who understands.