"Where are you going?"
Translation:Куда ты идёшь?
Because of what the actual english sentence is.
"Where are you going?" is incomplete. Often in english, we drop parts of the sentence that are understood, especially prepositional clauses. That is what happened here.
The actual english should be "Where are you going TO?" or "TO where are you going?" (Someone might say we can't end an english sentence with a preposition; that is not true, it is perfectly acceptable).
Где is "where," and куда is "to where".
When we ask "Where are you from", we simply say "Откуда ты?". Now when we ask "Where are you going?", why can't we say "Куда ты?"? In both situations, obviously the person must have moved/traveled from one place to another, so in principle either both should involve a verb of motion or both shouldn't.
Is this shortened phrase ever acceptable? I am fine with the answer "This is just the way Russian is", if that's the ultimate reason.
You don’t. «Собира́ться» is the meaning used for 'going to' when it's followed by a verb (я собира́юсь пообе́дать — I'm going to have a lunch). But in this sentence, 'going' is used in a different meaning.
It’s like in older English:
- Куда ты идёшь = Where art thou going? (singular),
- Куда вы идёте = Where are you going? (plural; but can be used when referring to one person too).
You should always use the pronoun with the correct verb form: you CANNOT say a form with a mismatching verb and pronoun:
- *Куда ты идёте? ‘Where are thou going’ — incorrect, ты ‘thou’ is not used with ‘are’ or «идёте», only with ‘art’ or «идёшь».
- *Куда вы идёшь? ‘Where art you going?’ — *incoirrect, вы ‘you’ is not used with ‘art’ or «идёшь», only with «идёте».
So, the verb changes depending on the pronoun you use. If you use «ты», you should use «идёшь» (and not «идёте»).
Both in Russian and in English, people started using the plural form to address a single person. So, ‘you are going’ and «вы идёте» are technically plural forms. But they can be used when speaking to one person. It is like ‘you’re so important to me, I value your opinion more than opinion of just one person’.
In English, this process reached its end, when people started being polite with everyone. So ‘thou art going’ is dead now.
In Russian, this process only happened partially. «Вы идёте» can be used for politeness. But «ты идёшь» is still alive and well. We use «ты идёшь» in contexts when we don’t need excessive politeness: when speaking to our friends whom we perceive as peers, when speaking to family members or when speaking to small children.