"लड़कियाँ सैंडविच खाती हैं।"
अनुवाद:Girls eat sandwiches.
The usage of articles "a, an ,the" in English follows some basic rules in Grammar. Here is an overview of the usage of articles - Article Explanation. In your sentence, the use of a signifies a single sandwich. If we wish to express that girls can eat any sandwich, we can drop the article.