what is the proper vert

I have seen 2 ways of translating the following.

Please respond.    Por favor RESPONSDA or RESPONDE.

which is correct

As soon as you use please, you are speculating in a formal way. The correct use is “responda” which is formal subjunctive. The word please automatically turns it into a request and not a command, so the indicative no longer works here, hence why responde is incorrect. Thanks Spanish!

¡Hola Ken3883 y Rainman!

You're absolutely on the right track to say that Por favor responda is formal, Rainman, but there's actually something a little different going on here.
 

In this sentence, responda is the usted “you (formal)” imperative form of the verb responder “to answer / to respond,” and it's being used to give a polite command: “Respond.” The formal usted imperative just happens to use the same verb form as the present subjunctive. (You can find a full explanation about these imperatives in Lesson 17.6: “Getting Bossy (Formally and in Groups)”). 

Responde, on the other hand, is the tú “you (informal)” imperative form of the same verb, and it's being used to give the same command in a more familiar register. (Positive tú imperatives are first introduced in Lesson 12.6: “Telling People What to Do,” and are revisted together with negative tú imperatives in Lesson 17.5: “Getting Bossy (Informally).”)

So both responda and responde are actually correct - it just depends on who you're talking to! You should say Por favor responda “Please respond” when you're talking to someone you don't know well, and Por favor responde “Please respond” when you're talking to someone you do know well.

Do let me know if any of this is unclear, or if you guys have any further questions!

Saludos,

Liss