Purpose: um … zu / damit

You can already say why something happened with weil. This lesson is about what something is for — the purpose, the goal. German has two ways to say it, and choosing between them is a single, mechanical test.

Same subject in both halves → um … zu. Different subjects → damit.

That is the entire lesson. Everything below is practice at spotting which one you have.

Start with um … zu. It has no subject of its own — it borrows the one from the main clause — and the zu sits directly in front of the infinitive at the end.

…, um · (the rest) · zu + infinitive (end). No second subject, ever.

• Ich lerne Deutsch, um in Deutschland zu arbeiten.

  I am learning German in order to work in Germany.

  I learn, I work — same person

• Ich gehe früh schlafen, um morgen fit zu sein.

  I go to bed early in order to be fit tomorrow.

• Ich spare Geld, um ein neues Auto zu kaufen.

  I am saving money in order to buy a new car.

With a separable verb the zu climbs inside the word: ein-zu-kaufen, an-zu-rufen. It looks odd and it is completely regular.

• Ich gehe in die Stadt, um einzukaufen.

  I am going into town in order to shop.

  einkaufen → einzukaufen

• Ich stehe früh auf, um pünktlich anzukommen.

  I get up early in order to arrive on time.

Now damit. Use it when the second half has its own subject — you are doing something so that somebody else can do or know something. It is an ordinary subordinate clause: subject, then the conjugated verb at the end, exactly like weil and dass.

• Ich spreche langsam, damit du mich verstehst.

  I speak slowly so that you understand me.

  I speak, you understand — two subjects

• Ich schreibe dir, damit du Bescheid weißt.

  I am writing to you so that you know.

• Ich erkläre es noch einmal, damit alle es verstehen.

  I am explaining it again so that everyone understands it.

Compare the pair directly, because the English is nearly identical and the German is not.

• Ich lerne Deutsch, um meine Kollegen zu verstehen.

  I learn German in order to understand my colleagues.

  I learn, I understand → um … zu

• Ich spreche langsam, damit meine Kollegen mich verstehen.

  I speak slowly so that my colleagues understand me.

  I speak, they understand → damit

One last thing worth knowing: damit is always allowed. If both subjects are the same you may still say damit ich …, it is simply heavier. um … zu is the natural choice when you can use it.