This is the 2nd most frequent Bulgarian word.
да
"Да" in Bulgarian means "yes" in English.
Here, 'да' is used as a particle introducing the verb in the infinitive form.
Искам да отида на кино.
I want to go to the cinema.
Here, 'да' is used to strengthen the suggestion introduced by 'нека'.
Нека да направим това заедно.
Let's do this together.
In this sentence, 'да' is used as an affirmative response, meaning 'yes'.