Titmaus, you are correct in your knowledge of English punctuation. In English we need to have a conjunction here such as "and." Alternatively, a semicolon could be used. Many people have not learned the rules regarding commas and think that it is okay to simply throw commas in wherever their voices pause in phrasing. They are mistaken. There are ten rules allowing commas, and when a sentence does not present any of these reasons, then probably a comma should not be used. Here they are: http://grammar.ccc.commnet.edu/GRAMMAR/commas.htm

To answer your question, Russian is a different language and has its own set of rules. It is perfectly acceptable to use a dash in Russian as is shown in this example.

personally my logic tells me that neither и in Russian, nor "and" in English would work, because they would change the meaning of the sentence, which is: "Tim is a dog, (therefore) it doesn't understand Russian". That's just me anyway.

Hey! The case of this word here is in accusative case. As a learner of another Slavic language I understand you so much how difficult it is to choose between genitive and accusative cases.

Okay, there's one simple rule for russian language: If the word has a meaning of "whose?" - you should use the genitive case.

We can't apply this rule for our situation, let alone the questions for the both cases are same.

Therefore, we should use accusative case and our sentence will look like this: Он не понимает (what? Can't say whose) русский язык.

But if we're speaking about a person or a situation, we can also use a question "whom?" Ex.: Я не понимаю (whom?) эту девочку (I don't understand the girl)

Good luck with russian! ❤

Да. Собака она, существительное женского рода. Но кличка собаки Тим, значит в этом случае это пёс, мальчик, кобель.