Learn German: Wir suchen das Büro. - We are looking for the office.

Büro is the direct object in this sentence (it is the thing being looked for), which puts it in the accusative case. However, because Büro is a neuter noun, its definite article stays das. In the accusative case, only masculine nouns change their article (from der to den).

Why isn't there a word for "for" in the German sentence?

The German verb suchen translates to "to look for" or "to seek". It already has the meaning of "for" built right into it! You never need to add a word like für; you just put the object you are looking for directly after the verb.

The English translation says "are looking for". Why isn't there a German word for "are" in this sentence?

German doesn't have an "-ing" form (the present continuous tense) like English does. "We look for" and "We are looking for" are both expressed exactly the same way in German: Wir suchen.

Why does the verb suchen end in -en here?

The verb needs to match the subject of the sentence, which is wir (we). In the present tense, regular verbs used with wir always end in -en, meaning they look exactly like their standard dictionary form.