Learn Dutch: Het appartement waarin de studenten wonen, is te klein. - The apartment the students live in is too small.

In Dutch, when you combine a preposition (like in) with a reference to an inanimate object (like het appartement), you cannot say 'in dat' or 'in wat'. Instead, you must use waar + the preposition. So 'in which' becomes waarin (literally 'where-in').

Can I drop waarin like we do in English ('The apartment the students live in')?

No, in Dutch you can never skip the connecting word in a relative clause. You must always include it, meaning you literally have to say 'The apartment in which the students live.'

Why is the verb wonen placed at the end of the first part, right before the comma?

The phrase waarin de studenten wonen is a relative clause, which means it gives more information about the apartment. Relative clauses follow the same word order rules as subordinate clauses: the conjugated verb is pushed to the very end of the clause.

Why does the main verb is come right after the comma?

This is the Dutch 'verb second' (V2) rule in action! The entire first chunk of the sentence (Het appartement waarin de studenten wonen) acts as one giant subject. Because that whole subject is in the first position, the main verb (is) must take the second position immediately after it.