Spanish - Abbreviations
is it usual in spoken spanish to say "yo soy" or only "soy" and in general are the pronouns often left out in the spoken language?
Technically that isn't an abreviation but an omision of the article.
As Lorel pointed there's no difference gramatically but we --at least in Mexico and Latin America- tend to omit the article a lot in everyday conversations because when the verbs or context of the sentence already indicate the subject -or article- the subject or article is just redundant.
If you're narrating an history it makes more sense to use the articles but for personal interaction you'll notice we drop the articles a lot.
Just a little suggestion/addition (and maybe it is a question too by a non-native speaker:)) to SmashCookie's pretty good explanation: Use the article, if you "feel" it's need in any way. So you can emphasize the person(s) - the waiter brought the bill and you say "Yo pago".... Or maybe "¿A dónde vais?": "Yo quiero pasear por la ciudad, pero ella quiere comprar zapatos."
And of course - like said - there may be differences between countries... (Last week we were in Spain, usually we have vacations in Mexico, there are more differences than "zumo" and "jugo" or the use of "vosotros" ;))
Any hints are welcome :)