"se", "daba" and "estaba"
I am teaching myself Spanish at the moment. I have got stuck on the word "se" which I've never encountered before.
Examples:
Se puso triste cuando su perrito murio.
Sara se puso contenta porque habia aprobado el curso
What does "se" mean here? To become?
Then in these examples:
"Daba pena verle tan deprimido." Can I change it to "Se puse pena verle tan deprimido" ? Any differences in the meanings?
and "Estaba aburrido asi que se fue al cine", is it possible to change it to "Se puso aburrido, asi que se fue al cine"
Please help, I am so confused here!!!!!