That's right, it is a bit hard for English-learners. But the rule is very straightforward. For the most part, a singular noun is never alone in English. If it doesn't have a noun determiner "this, that, one, any, some, no," then it needs an article "a, an, the." If you follow this rule, you can't go wrong, except for uncountable nouns like water, sand, mud, gold, food, wood,, etc. They can be alone, without noun determiner or article.
Just for everyoneʼs information, “articles” are just a subset of “determiners” and they include “a”, “an”, and “the”. The rule of thumb is that countable nouns always need determiners of some sort unless they are plural. There are a few corner cases where that isnʼt true, but itʼs much easier to be told that you donʼt need a determiner in this specific case instead of being told you need a determiner in all of these cases.
This is the correct pronunciation. О and а are only distinguished when they are stressed. In «э́то», «э» is stressed (and not «о»). If you pronounce «о» in a 'more accurate' way, this would come off as something dialectal.
Pronouncing unstressed «о» as «о» is called «о́канье».
Such pronunciation is imitated in Alla Pugacheva's song «Посидим, поокаем» '[Let's] sit down and talk, pronouncing unstressed «о» as «о»'. The song is about a girl who invites a guy to a date and dreams about seeing him, but he never comes. In this song, the suggestion to pronounce «о» as «о» probably indicates they could have an informal conversation, not trying to speak in a 'standard', 'correct way'.
A bench with the name of this song was istalled was installed in Vologda, where a dialect with «о́канье» is traditionally spoken:
However, standard Russian is based on the Moscow and Saint Petersburg pronunciation, where unstressed «о» is pronounced as «а» (a reduced «а», actually; but the reduction is optional). Pronouncing unstressed «о» as «а» is called «а́канье», and this is the standard pronunciation.