"Proper" vs "Casual" conversation.
I used to date a girl from mexico (partially sparking my interest in learning Spanish) and one thing she told me was that certain words are only used in proper settings like business meetings. But if you're just hanging out with friends than you wouldn't use that kind of language or you'd sound weird.
One such example was beber vs tomar.
Is this the only case or does it apply to other verbs (or words in general)? If so how do I go about learning more, or all, of these cases?
It can be highly regional too. Tomar is used for to drink a lot in Mexico, but not everywhere in the Spanish-speaking world. In Mexican telenovelas, I've been learning lana for money instead of dinero (or wool which is what lana actually means).
It's better to be able to use and understand more formal speech, then you can learn the informal words, expressions, and slang as you go depending on where you are. People will tell you right away if they think you are using words that are oddly formal. They might laugh at you, but it's all in good fun and it's part of the process of learning.
"Sí, ¡dale!" is best suited for casual situations, not a meeting with a client. Learned that one the hard way. lol
And of course there's "tú" vs "usted" and the associated conjugations.
I was also taught to prefer asking "Otra vez, por favor" over "¿Qué dijiste?" if I didn't hear something the first time, and to avoid the more brusque "¿Qué?"
It's better to be a little more formal than not, especially in a business situation or with people that you don't know. When I worked in a shop many years ago, a spanish-speaker taught me that I should politely say "Permiso" (excuse me) instead of "ándale," which is what I had learned when I was much younger. Luckily, he was good-natured and realized that had been joking a bit.
How would you go about learning more?
You will get many clues from news programs, telenovelas and films. Like Elizabeth261736 stated it is also regional. What is acceptable or daily used in México is not used in Costa Rica, Colombia or España. I live in Nueva York (en los estados unidos) and there are many diverse Spanish speakers, and many that I encounter use a very loose, casual style. So I learned some of them just by eavesdropping and paying attention but I also know the proper way because I watch the news when I can. News reporters are obviously not allowed to use slang.
Yes, people will quickly let you know that you don't need to be formal. Eventually as you get more exposure, you recognize that some abbreviated words are often used like 'toy instead of estoy or 'pa instead of para. If you want a fun and some what difficult guide to informal speech just listen to a lot of hip hop music particularly Calle 13.
In all languages there are formal and informal or more relaxed ways of speaking. In this case it's better safe than sorry, especially if you aren't a native speaker and no one expects you to sound native. In business meetings, writing, in a shop or in the airport, etc, it might sound a little off to be extra formal, but that's better than risking to offend someone by being too informal.
Spanish is extremely regional. Some words have very different meanings in different countries, so you will never learn all there is to learn. I'm from Latin America (Uruguay), and sometimes is hard even for us to understand native Spanish speakers from other countries. I have a really hard time with people from Chile.
For example, "coger" in Spain is "to take". In Uruguay "coger" is "to ❤❤❤❤", and is never used as "to take". So Spanish can vary a LOT.
In a language as complex as Spanish it's much more important to learn grammar and verb tenses than to worry about specific words. Master the subjunctive, then worry about contractions and slang.
And watch movies. Lots of movies. Watch an Almodovar movie and then an argentinian movie (Historias salvajes is really god, and there's lots of swearing, that's always fun to learn).