Learn Russian: Она работает дома, но мы едем в офис. - She works at home, but we are going to the office.

Both а and но can translate to 'but', but they do different jobs. We use но to show a stronger contrast or contradiction, like pointing out that we have to commute even though she gets to stay home. If you used а, it would simply compare two neutral facts: 'She works at home, whereas we go to the office.'

Why does 'at home' use one word (дома) while 'to the office' uses two (в офис)?

дома is a special adverb that means 'at home' (your current location). It already includes the idea of 'at', so you do not need a preposition. For the office, we are showing direction or movement towards a destination, so we use the preposition в (to/into) followed by the noun офис.

Why do we use едем for 'we are going'? Can I use идём?

You use едем when you are going somewhere using transport, like a car, bus, or subway. You would only use идём if you wanted to emphasize that you are specifically walking to the office on foot.

Since we are moving to a destination, shouldn't the ending of офис change?

When showing direction to a destination with в, Russian uses the accusative case. However, for inanimate masculine nouns like офис (objects or places, not living beings), the accusative form is exactly the same as the basic dictionary form. Because of this rule, the word does not need to change!