tečno

1. (of a language) fluently
   Posle dve godine već tečno govori srpski. — After two years he already speaks Serbian fluently.
   Tečno čita i piše na tri jezika. — She reads and writes fluently in three languages.
2. smoothly; fluidly
   Razgovor je tekao tečno i opušteno. — The conversation flowed smoothly and easily.

As an adverb it is mostly used with verbs like govoriti, pričati, čitati.