When to use "в" and when not to use?

Question

I'm learning Russian. In the tutorial they say:

Витя дома

meaning

Victor is at home

Also they say:

Витя в Токио

Meaning

Victor is in Tokyo

So why are they using в at one place and not using в at another place?

Answer 1

Дома is "at house", в доме is "in the house" - this just should be memoized. There are no other such cases, since "дома" is quite special word in this aspect.

As a sidenote, there's also form на дому (and this is actually real example of locative) - it's about having some work-related practice at home, like "Врач принимает на дому".

Answer 2

'Дома' is a locative adverb, much like e.g. 'наверху' or 'внизу', and, as such, does not require a preposition. Actually, in colloquial English, one can also say 'I'm home' to state the fact of arriving there, also making the word 'home' an adverb.