"English actresses are pretty."
Translation:Az angol színésznők szépek.
The translation should be correct with and without "the" as well.
An article is always needed in front of the subject if the predicate is a noun or an adjective. "Angol színésznők szépek" is not correct. But once the article is there, without context you can't tell anymore if the sentence refers to English actresses in general or a specific group of them, so both translations are correct.
Either way is fine. Without "the", it is a general statement. With "the", it could be a specific group of people. The Hungarian version needs "Az" in either case.
May I say "EGY angol színésznők szépek"? Or "egy" can be used only in singular form?
Just like in English, the indefinite article can only be used in the singular. Even more so in Hungarian since "egy" also means "one".