I learned from my teacher that indefinite articles (un, una) are used only before modified nouns, that is nouns followed by adjectives.
Does this apply to definite articles (el, la)?
Another situation is when you use "llegar a ser" in place of just ser. This is when you don't have to use indefinite article.
indefinite articles (un, una) are used only before modified nouns, that is nouns followed by adjectives.
I think that this is not true.
A modified noun with "una":
una casa roja
(=a red house)
An unmodified noun with "una":
una casa
(=a house)
Both sentences are perfectly valid in Spanish. And, regarding modified and unmodified nouns, definite articles behave like indefinite ones, there is no difference:
A modified noun with "la":
la casa roja
(=the red house)
An unmodified noun with "la":
la casa
(=the house)
I am guessing what your teacher meant to say was that after the verb ser (or one of its forms) you only use an indefinite article with a noun that is modified with a descriptive adjective or an adjectival phrase. For example: Ella es doctora. (She is a doctor.) Ella es una doctora que vive en el campo. (She is a doctor that lives in the country) or Ella es una doctora muy buena. (She is a very good doctor.) That rule of only using the indefinite article before a modified noun only applies with the verb ser. This was taken from information presented in a college grammar workbook written by Conrad Schmitt (Schaum's outline series) through the company McGraw Hill. I don't know the copyright year but it would have been in the 1980s.
I'm not a language teacher nor a linguist but I think your teacher is wrong or you missunderstood something.
I've seen a dog
He visto un perro
Perro is not a modified noun (appart from being indefinite) there but it is not a particular dog so we use indefinite article.
I've seen the (same than last week) dog
He visto el perro
For the second situation you mention, I don't understand your question, sorry. But I can say that llegar a ser (that is, become) accepts both definite and indefinite direct objects:
I've become the best lawyer of New York
He llegado a ser el mejor abogado de Nueva York.
I've become a good lawyer at New York.
He llegado a ser un buen abogado en Nueva York.
indefinite articles (un, una) are used only before modified nouns, that is nouns followed by adjectives.
The usage, as well as the difference in usage, of definite and indefinite articles has no dependency on whether or not the following word is a modified noun or a noun. They're both the same thing, a noun.
Still, it is a noun at the end of the day.
Definite articles, like el and la , are used to define a particular object or subject. It conveys singularity, while the indefinite articles: un and una suggest there are more than one of this subject/object.
These rules are exactly the same in English.
