Using an article before the noun...

Question

I am not sure when to use an article (especially indefinite article) before a noun. For an example:

Estoy a régimen muy estricto. - I am on a very strict diet.

"regimen" is not a rank, nationality, or occupation, so why doesn't it have the indefinite article "un" before it? Help please! Thanks!

Answer 1

I cannot give an answer as to why you would not use the indefinite article here only to say that it is similar with professions where the idefinite article is omitted as in, soy médico.

I just know that the indefinite is also omitted here, "estar a o hacer régimen" is to be on a diet.