a and the
When to use 'un' or 'una' and when to use 'la' 'las' 'los' and 'las'? Is there a set room for it? For instance, in English there is I am an engineer, but in Spanish the 'an' is dropped - Soy ingeniero.
right. In Spanish: Soy ingeniero. - without an article. Like: You are 'TheNarnianDragon' and I am Abendbrot.
Is there a rule, why there has to be an 'a' in English. -not really. You have to learn that the phrase 'I am [profession]' does not need an article.
I think (but I don't know), in English you use an 'a' in front of a profession because the English language says 'You are one of a group'. You are a baker. A baker of many.
In the Spanish language a profession is like a character or a name. It belongs to you. Tú eres Julia. Tú eres ingeniera.
Another difference is that in Spanish, you say "la boca", "la cabeza", "el pie" when talking about your body parts instead of "my mouth", "my head", or "my foot". "My head hurts" is "me duele la cabeza" or literally "the head hurts me".