Learn Russian: Где яблоко? Оно там. - Where is the apple? It is there.

In English, we use "it" for almost all objects. In Russian, objects have a grammatical gender. Because яблоко (apple) ends in -о, it is a neuter noun. Оно is the neuter subject pronoun, so it perfectly matches the noun it replaces.

If we were asking about a feminine word like книга (book), how would the second sentence change?

You would have to change the pronoun to match the gender of the new noun. Since книга is feminine, you would use the feminine pronoun она (she/it) and say Она там.

Why is there no word for "is" in Где яблоко or Оно там?

In the present tense, Russian simply drops the verb to be (am, is, are). So the sentences translate literally as "Where apple?" and "It there." You only use a word for "is" if you are talking about the past or the future.

Why do the letter o's in оно and яблоко sound more like an "a" when spoken?

This happens because of a Russian pronunciation rule called vowel reduction. When the letter о does not have the spoken stress on it, it sounds like an "a". In оно, the stress is on the last letter, so the first letter sounds like "a". In яблоко, the stress is on the first letter, so the unaccented o's sound like "a".