Learn Portuguese (Portugal): Diz-lhe que me traga o livro.

Traga is the present subjunctive form of trazer (to bring). We use the subjunctive here because the first verb, dizer, is being used to give an indirect command or request (tell him to do something). When you tell someone to do something using que, the following verb must be in the subjunctive.

Why is the pronoun after the verb in Diz-lhe but before the verb in me traga?

In European Portuguese, pronouns generally attach to the end of the verb, which is why we have Diz-lhe (an affirmative command). However, certain words pull the pronoun to the front of the verb. The conjunction que is a strong magnet word, so it pulls the pronoun me right in front of the verb, creating que me traga instead of using a hyphen.

Is the command Diz formal or informal?

It is informal. Diz is the tu form of the imperative (command) for the verb dizer (to say/tell). If you were speaking formally to someone using the você form, you would say Diga-lhe que me traga o livro.

What verb does traga come from?