Why in some phrases there is the voice and in others no?
The voice is only there for sentences in Esperanto
Why not "Sofia estas la virino"? Why leave out "la"?
La means the. The sentence is "Sofia is a woman."
That would mean Sofia is the woman
make sense for sure, but it's not the correct translation
Which is correct Sofia estas virino or Sofia virino estas
"Sofia estas virino" is correct
Both "Sofia estas virino." and "Sofia virino estas." are correct.
Word order in Esperanto is not completely "free", but still rather flexible compared to a bunch of other languages.
I always forget if the article doesn't translate into "la virino" when it arranges together with the auxiliary verb so that the sentence is gonna be "Sofia estas la virino". And it's incorrect
I don't know what you mean by "arranges together with the auxiliary verb". The verb has nothing to do with it. Esperanto only has "the"--"la". It does not have "a/an".
I am wondering if I can say "Sofia estas unu virino"? I suppose "unu virino" which means "one woman" has a big difference to "a woman", but I don't know how exactly.
Sofia is a woman. She's not a boy or a lamp.
Sofia is one woman. Don't ask her to do the work of three people simultaneously.
Just like 'a' and 'one' arent necessarily equivalent in english.
In esperanto, there is no word for 'a', whenever we in english say 'a' in esperanto they say nothing