This is the 487th most frequent Slovenian word.
nista
The Slovenian word "nista" means "you are not" (second person plural) or "they are not" (dual form).
Here, 'nista' is used as a negative past tense auxiliary verb for 'vi' (you plural) working with the verb 'šla' to say 'did not go'.
Nista šla na tržnico včeraj.
You both did not go to the market yesterday.
In this sentence, 'nista' functions as a negative form of the verb 'biti' (to be) in the present tense for 'vidva' (you two).
Nista toliko podobna, kot sem mislil.
You two are not as similar as I thought.
In this usage, 'nista' is again from the verb 'biti' (to be) in present tense for dual, used in a question.