When is 'to' translated with “um zu”, when with “zu”

Question

The infinitve marker "to" can not simply be translated with "zu". Apparently in most instances it translates with "um zu", wheras in other instances "zu" alone is used.

¹ "Del" is the key to press.
- "Entf" ist die zu drückende Taste.

² "Del" is the key to delete letters.
- "Entf" ist die Taste, um Buchstaben zu löschen.

³ Press "Del" to delete letters.
- Drücken Sie "Entf", um Buchstaben zu löschen.

⁴ It is a bad idea to press "Del".
- Es ist keine gute Idee "Entf" zu drücken.

Are there any rules of when to use "zu" and when to use "um zu"? Is it a good style to translate "to" with "um zu" or is it be more elegant to avoid the infinitive (e.g. in ² "Die "Entf"-Taste löscht Buchstaben")?

Answer 1

So the "um zu"-part has already been explained. When you can replace your "to" by "in order to" or rephrase to "because I want to ... " then "um zu" is the correct choice.

What I would like to add is a method to check for a simple "zu". Imagine a room full of people. You open the door and you say the first part of your sentence and then you leave. If you can do that without confusing everyone, it is probably "um zu" but if your sentence does NOT make any sense by itself, it is going to be "zu".

Example

"I am trying."

Unless you have discussed whatever you are trying before, this statement is nonsense. It needs the "to"-part as completion and thus is followed by just "zu" in German.

"Ich versuche zu lesen."

"I am trying" is not complete. Now let's look at a sentence that is.

"I am going to the kitchen, (to get a beer)."

People might wonder who cares but still they are well-informed about your plans.

Ich gehe in die Küche, um ein Bier zu holen.

So when the first part doesn't make sense by itself, the "to" is mostly translated to "zu".

There is one more thing that needs to be mentioned. If you want to connect a modal verb like können or wollen with an infinite form, the "to" just disappears. For instance:

I want to eat.
Ich will essen.

I have to go.
I must go.
Ich muss gehen.

This also applies for gehen by the way.

"I go to the supermarket to buy milk."

According to the "in order to"-rule the translation should be:

"Ich gehe in den Supermarkt, um Milch zu kaufen."

This is correct but the following is also proper:

"Ich gehe in den Supermarkt Milch kaufen."

So here the "to" disappeared as well.

Blockquote

Answer 2

Expanding on @Stefan Walter's statement...

"Um" with "zu" always starts a subordinate clause describing an aim or a purpose.

The construct um [etwas] zu [infinitiv] more literally translates to the English phrase in order to [do something]. It just happens that the phrase isn't terribly common in English. "Zu" is used alone for simply convey an infinitive verb. Following this definition, the meanings of your examples 2 and 3 change slightly (though you don't have to change the English wording):

2 "Del" is the key [to press - implied] in order to delete letters. - "Entf" ist die Taste, um Buchstaben zu löschen.

3 Press "Del" in order to delete letters. - Drücken Sie "Entf", um Buchstaben zu löschen.

If there is no object of the action, you don't need the "um".

Answer 3

"Um" with "zu" always starts a subordinate clause describing an aim or a purpose. This is clearly not the case in ¹ and ⁴.

³ sounds good. ² does not, since one would expect the clause "um Buchstaben zu löschen" to refer to an action and not to an object. I suggest:

"Entf" ist die Taste, mit der man Buchstaben löscht (löschen kann).

Of course, in a manual, it might be appropriate to use "Sie" instead of "man".