Four words that change nothing
und, aber, oder and denn join two complete main clauses. The second clause is built as if it stood alone: subject first, verb in second position.
What each one means
und: adds
Like English and. It also joins single words: Brot und Milch.
Ich koche und du deckst den Tisch.
I'll cook and you set the table.
aber: contrasts
Like English but.
Ich möchte gern mitkommen, aber ich habe keine Zeit.
I'd like to come along, but I don't have time.
oder: offers a choice
Like English or. At the end of a statement, oder? works as a question tag, like right?
Nehmen wir den Bus oder gehen wir zu Fuß?
Shall we take the bus or shall we walk?
Wir treffen uns um acht, oder?
We're meeting at eight, right?
denn: gives the reason
It means because. The reason always comes second, so denn never starts a sentence.
Wir nehmen ein Taxi, denn es regnet stark.
We're taking a taxi, because it's raining hard.
Commas
Before aber and denn a comma is required, even where English would leave it out. Before und and oder between two full clauses it is optional.
Es ist spät, aber ich bin noch wach.
It's late, but I'm still awake.
comma required before aber
Ich bleibe hier, denn ich bin müde.
I'm staying here, because I'm tired.
comma required before denn
Saying the subject only once
If both clauses have the same subject, und and oder let you say it once. There is no comma then. Both verbs keep their normal form.
Ich stehe auf und mache Kaffee.
I get up and make coffee.
Sie wohnt in Köln und arbeitet in Bonn.
She lives in Cologne and works in Bonn.
