No, it's rarely mandatory except in certain question forms or with specific negative adverbs. However, it is stylistically 'mandatory' if you want to sound like a native speaker.
It doesn't change the basic facts (who did what), but it changes the 'pragmatic' meaning—what the speaker thinks is most important or surprising.
Yes, but it's more common in formal writing. In speech, we usually put the reason after the main clause.
Meaning is identical. 'Pentru că' is neutral/informal, while 'deoarece' is more formal and preferred in writing.
No, in Romanian it is almost always required, unlike English.
Use 'să' for wishes, commands, or after verbs like 'a vrea'.
