wo
where
An interrogative adverb used to ask about a place or position.
Wo wohnst du?
Where do you live?

where
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where
An interrogative adverb used to ask about a place or position.
Wo wohnst du?
Where do you live?

'Wo' looks like the English 'who', but it always asks about a place (where?). For a person, German uses the question word 'wer' (who).
Wo ist der nächste Supermarkt?
Where is the nearest supermarket?
As a question word, 'wo' introduces a W-question with the verb in position 2. As a relative adverb or conjunction, it introduces a subordinate clause in which the conjugated verb comes at the end.
Wo beginnt der Wanderweg?
Where does the hiking trail begin?
In spoken language, 'wo' is often used as a universal relative pronoun instead of 'der', 'die', or 'das'. This use is considered stylistically poor in written language and formal contexts.
Kennst du den Film, wo ein Hund die Hauptrolle spielt?
Do you know the film where a dog plays the main role?
As a correct relative adverb, 'wo' must refer to a place or a location expression in the main clause. Typical reference words are 'Stadt' (city), 'Haus' (house), 'Platz' (place), 'Land' (country), or adverbs like 'dort' (there).
Wir fahren nach Berlin, wo meine Tante wohnt.
We are going to Berlin, where my aunt lives.
the
'der' and 'wo' can both introduce a subordinate clause. 'der' is a relative pronoun and here refers to a masculine noun, for example to 'der Mann' (the man). In Standard German, 'wo' refers to a place, for example to 'der Park' (the park) or 'die Stadt' (the city). They can be confused because in colloquial speech you sometimes hear 'wo' even when no place is meant. In good Standard German, however, you do not say 'der Mann, wo ...', but 'der Mann, der ...'. Remember: for people and things, use the appropriate relative pronoun; for places, 'wo' can be natural.

Im Urlaub zeigt dir Lisa den Strand, wo sie als Kind oft gespielt hat.
On vacation, Lisa will show you the beach where she often played as a child.
Der Arzt, der heute Dienst hat, spricht zuerst mit der Patientin.
The doctor who is on duty today will speak with the patient first.