Learn Hardcore Russian: Ты интересуешься историей или предпочитаешь науку? - Are you interested in history or do you prefer science?

The -ся is not an extra word but part of the verb интересоваться, which means “to be interested in.” It is a so-called reflexive ending, but in this verb it doesn’t make the action reflect back on the subject. Instead it turns интересовать (“to interest someone”) into интересоваться (“to become interested”).

Why is историей in the instrumental case rather than accusative?

The verb интересоваться always takes its object in the instrumental case. You ask интересуешься чем? (“interested in what?”) and answer историей. So:
• интересуешься + instr. (историей)
This is just how Russian governs cases for this verb.

Why is науку in the accusative case instead of instrumental?

The verb предпочитать (to prefer) is a transitive verb that takes a direct object in the accusative case. You ask предпочитаешь что? (“you prefer what?”) and answer науку.

Why isn’t the particle ли used here to form the question?

The particle ли is used for simple yes/no questions or indirect questions (e.g., интересуешься ли ты?). In our sentence you have an alternative question because you’re giving two choices—history or science—joined by или (“or”). In alternative questions, или replaces ли, so no separate ли is needed.

How would I make this sentence polite using вы instead of ты?

You simply switch the verb endings from the second person singular to the second person plural/formal:
• вы интересуетесь историей
• или предпочитаете науку
So the polite version is:
Вы интересуетесь историей или предпочитаете науку?

Why is ты only used once at the very beginning? Does it apply to both verbs?

Yes. The sentence has two coordinated verbs sharing the same subject ты. Once you state ты, it applies to both интересуешься and предпочитаешь. For emphasis you could repeat ты, but it’s perfectly normal in Russian to mention it only once.

Is this a yes/no question or an alternative question? How can I tell?

This is an alternative question, not a yes/no question, because it presents two options joined by или. You expect an answer like:

“I am interested in history.”
“I prefer science.”
“Both.”
A yes/no question would use ли or just intonation without offering choices.

How would I answer this question in Russian? Can you give examples?

Here are some common responses:
• Я интересуюсь историей. (I am interested in history.)
• Я предпочитаю науку. (I prefer science.)
• Мне интересна история, но больше нравится наука. (I find history interesting, but I like science more.)
• И то, и другое. (Both.)

Where is the stress in интересуешься and предпочитаешь?

• интересу́ешься – stress on су́
• предпочита́ешь – stress on та́
Paying attention to stress helps with correct pronunciation.

Can I drop ты in informal speech and still be understood?

Yes. Russian verbs carry person/number endings, so you can omit the subject pronoun in informal contexts. The question becomes:
Интересуешься историей или предпочитаешь науку?
It’s perfectly natural in casual conversation.