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)?

Okay. I'm really confused. How did I get half of these questions wrong. I know Spanish. But yet, I got some of these questions wrong.

Why do some of the verbs get tildas?

se lo quiero comprar becomes comprárselo

and 

se lo estamos decorando becomes estamos decorándoselo

The answer says:  Pasó dos días sumergido en plena naturaleza sin que nadie lo molestara enviándole mensajes o llamándolo.

Can you also say: Pasó dos días sumergido plena naturaleza sin que nadie le molestara enviándole mensajes o llamándole.

Is this  Spain vs Latin America  grammar?    Also, in general, I thought molestar was an inverted verb like gustar and required "le."

Re les vs los below.. les is incorrect because ver is a transitive verb, correct? 

She hates cats. Not THE cats. Why "los gatos" then?

I don't understand why Indefinido is used in the example; "Nos gustamos desde el primer momento." It has a definite starting point, but the sentence implies a continuing action. In this case could Imperfecto also be used correctly?