Learn Hardcore German: Ich bin heute zu müde, um lange zu telefonieren. - I am too tired today to talk on the phone for a long time.

In zu müde, zu is an adverb meaning “too.”
In um … zu telefonieren, zu is the infinitive marker used in the um … zu + infinitive construction.

Is the comma before um required?

Yes. An infinitive clause introduced by um must be set off with a comma: …, um … zu …

Can I drop um and say Ich bin heute zu müde, lange zu telefonieren?

No. In the pattern zu + adjective, um … zu + infinitive, the um is required. Without um, the sentence sounds wrong or ambiguous.

Why is it um lange zu telefonieren and not um zu lange telefonieren?

um lange zu telefonieren = “in order to phone for a long time,” where lange (for long) is an adverb modifying the action.
zu lange telefonieren would mean “to phone too long,” with zu again meaning “too,” which changes the meaning.
Also, the infinitive marker zu belongs immediately with the verb: zu telefonieren.

Why lange and not lang?

As an adverb of duration, standard German prefers lange: lange telefonieren (“talk on the phone for a long time”).
Use lang after a specific measure: zwei Stunden lang telefonieren. Colloquial lang telefonieren exists but is less standard.

Can I say für lange to mean “for long”?

Generally no. Say lange on its own: … um lange zu telefonieren.
If you use für, you need a noun: für eine lange Zeit—but that sounds heavy here.

Why use telefonieren instead of anrufen?

telefonieren = “to be on the phone; to have a phone conversation” (often with mit + Dative: mit dir telefonieren).
anrufen = “to call (someone)” and takes a direct object: jemanden anrufen.
So “too tired to call you” is zu müde, um dich anzurufen, not … dich zu telefonieren.

Where does the infinitive zu go with separable verbs like anrufen?

Between the prefix and the verb stem: anzurufen (not “zuanrufen”).
Example: Ich bin zu müde, um dich anzurufen.

Can I say Ich bin heute zu müde zum Telefonieren?

Yes. zum = zu dem, and Telefonieren is a nominalized verb, so it’s capitalized.
To keep “for a long time,” you’d have to say zum langen Telefonieren, which is grammatical but a bit stiff; … um lange zu telefonieren sounds more natural.

Why is telefonieren lowercase here?

It’s a verb in an infinitive clause (zu telefonieren). Only the nominalized form is capitalized: zum Telefonieren.

Can I move heute to another position?

Yes:

Heute bin ich zu müde, um lange zu telefonieren. (fronted for emphasis)
Ich bin zu müde, um heute lange zu telefonieren. (placing “today” inside the infinitive clause) All are natural; the meaning stays the same with slight shifts in emphasis.

Who is the subject of the um … zu clause?

It’s implicitly the same as the main-clause subject (ich here). If you need a different subject, you can’t use um … zu; you’d restructure, e.g.:

Ich bin so müde, dass du besser nicht lange telefonierst. (different subject)
More formal: Es ist zu spät, als dass wir noch lange telefonieren könnten.

Is there an alternative to express the idea without um … zu?

Yes, the so … dass pattern:

Ich bin heute so müde, dass ich nicht lange telefonieren kann.
Same idea, but it states the consequence explicitly with dass and kann.

How would I say “I’m not too tired to talk for long”?

Ich bin heute nicht zu müde, um lange zu telefonieren.
Alternatively (neutral statement of ability): Ich kann heute lange telefonieren.

How do I include the person I’m talking with?

Use mit + Dative:

Ich bin heute zu müde, um lange mit dir zu telefonieren.
This keeps telefonieren (intransitive) and adds the partner with mit.

How do you pronounce ü in müde?

Round your lips as for “oo” but say “ee”: a front, rounded vowel. Try saying English “ee” while rounding your lips—müde sounds like that, roughly “MYOO-duh” (but with a short, unaccented second syllable).