“Smetti di” oppure “smettila di”?

Question

I have a question. I know that “to stop doing something” is *smettere di fare qualcosa*. Nevertheless I’ve heard Italian speakers add the pronoun *la* when using the verb in the imperative mood. So instead of "Smetti di fare questo!" I hear instead "Smettila di fare questo!" Is there a reason why the *la* should be there? It doesn’t seem to be doing anything. And why is it feminine (*la*) rather than masculine (*lo*)?

Answer 1

The 'la' adds emphasis to the imperative of certain verbs. For example, 'Smettila di stuzzicarlo!' can be translated simply as 'Stop teasing him!', but for added emphasis in the translation, one might say, "Just stop teasing him, will you?!"

The 'la' stands for 'quella cosa'.

Another example of an imperative with added 'la': Piantala di fare quella faccia!" - "Stop making that face (, will you?!)"