'soy' and 'sea' difference
When do we say ' yo soy' and 'yo sea'?
"Soy" is presente, "sea" is present subjunctive. You'd use "soy" to say "Soy de Inglaterra", or another fact about yourself. You'd use "sea" to express something that could/could not be real, for example "Cuando sea rica, iré a India" - when I'm rich I'll go to India, but I don't really know when I'll be rich, so I use the subjuctive. Another example: "Aunque sea desagradable, es muy generoso" - I expressed the opinion that he is not a nice person (although he "may not" be nice, he's very generous) so I used the subjunctive.
Corrections to my post would be welcome, as I think the subjunctive tense is extremely difficult to understand, especially for English speakers (subjunctive is not used much nowadays) and even French speakers who I also think don't use the subjunctive very often!
You forgot "Sea" is for imperative [formal] (being "sean" the plural form and "se" the singular informal form) = Se más cuidadoso-a/Sea más cuidadoso-a/Sean más cuidadosos-as(Be more careful). although there are some changes in the negative form for "se" = Don't be so rude -> No seas tan rudo, No sea tan rudo, No sean tan rudos