Spanish language Q&A Forum

When would be appropriate to use formal and informal second person?

For context, I'm planning to speak mostly with my Mexican family members, many that I'll be meeting for the first time or for the first time in many years. Most of them are older than me (I have one younger brother who speaks mostly Spanish). Would it be better to speak formally to the older family members, or informally because they're family?

Would you start formally with strangers (esp. those who are older) and then at some point be able to drop formality as you become closer to people, or as they express to you that it's okay (similar to Korean and Japanese)?

La narradora dice “las pilas” pero el texto dice “la pilas”, y el programa te corrige si escribes “las pilas” en tu transcripción. Me gusta esta expresión que no conocía antes. Creo que ya la entiendo, pero tal vez merezca una pequeña nota. ¡Gracias!

How in the hell is that "llegaremos"? Even with about 50 repeats I did not get that. Is she actually saying something else or how does that work? Always sounds like "digaremos" to me :-(

A little off topic, but consider:

1. donde, adonde/a donde

2. dónde, adónde/a dónde

and for that matter (or maybe especially for the case of), 

3. quizá, quizás 

Within each group the various options can be used interchangeably. But what factors influence the chosen form? For example do some people tend to use the same form all the time? Do people just randomly use all the forms equally? Do some localities tend to use one form more than others? Is there a pronunciation efficiency issue (similar to y and i  or  o and u, but not a hard-and-fast rule)?

I guess my questions especially apply to quizá versus quizás.