Learn Hardcore Hungarian: Ha muszáj, felhívlak telefonon, amikor hazaérsz. - If I must, I will call you on the phone when you arrive home.

It’s an ellipsis meaning “If it’s necessary / If I have to.” Hungarian often leaves out the infinitive after muszáj when the rest of the sentence makes it obvious. The full version could be: Ha muszáj, fel kell hívnom téged or Ha muszáj, felhívlak—both are natural. You can also add a dative to clarify who “must”: Ha muszáj nekem… (“If I must…”), Ha muszáj neked… (“If you must…”), etc.

How is muszáj different from kell?

Both express necessity, but:

muszáj feels a bit stronger or more categorical (“it’s a must”), and often appears as a standalone predicate: Muszáj mennem.
kell is more neutral and is the regular verb of necessity: Fel kell hívnom (téged). In your sentence, Ha muszáj ≈ “If necessary / If I must,” with a slightly reluctant nuance.

Why is there a comma after muszáj and before amikor?

Hungarian typically uses a comma to separate subordinate clauses or clause-like adverbials:

Ha muszáj, … (“If [it’s] necessary, …”)
comma before amikor: …, amikor hazaérsz. Even though Ha muszáj is elliptical, it functions as a subordinate condition, so a comma is standard.

What exactly does felhívlak encode?

It’s the verb felhív (“to call [up] by phone”) conjugated with the special 1st-person-subject + 2nd-person-object ending -lak/-lek. So felhívlak means “I call you” (or, in context, “I’ll call you”). Contrast:

felhívom = “I call him/her/it” (1sg definite, 3rd-person object)
felhívok = “I call (someone)” (1sg indefinite; you’d add a nonspecific object) Note: for “you (plural)”, you still use -lak/-lek: Felhívlak titeket = “I’ll call you guys.”

Can I say felhívok téged instead of felhívlak?

Avoid it. When the object is “you,” Hungarian strongly prefers the special -lak/-lek ending: felhívlak. The version felhívok téged sounds off or unidiomatic to most speakers unless used with unusual emphasis.

Why is it sometimes written together (felhívlak) and other times split (hívlak fel)?

The particle (preverb) fel- normally sits before the verb and is written together: felhívlak. It moves after the verb in certain environments:

Negation: Nem hívlak fel.
Focus in front of the verb: Telefonon hívlak fel.
Questions often do the same: Mikor hívlak fel?
Imperative: Hívd fel! With the future auxiliary, it separates: Fel foglak hívni.

Do I even need telefonon if felhív already means “call (by phone)”?

It’s optional. Felhívlak already implies a phone call. Adding telefonon (“on the phone,” superessive case) can emphasize the channel or make the medium explicit (e.g., as opposed to a video call or a doorbell). Both are perfectly natural:

Felhívlak, amikor hazaérsz.
Felhívlak telefonon, amikor hazaérsz.

Why telefonon and not telefonnal?

For means of communication, Hungarian typically uses the superessive: telefonon (“on the phone”), Messengeren, emailben. Telefonnal (instrumental “with a phone”) is possible in other contexts (“I’ll take a photo with a phone”), but for “call on the phone,” telefonon is the idiomatic choice.

Why is amikor hazaérsz in the present if it refers to the future?

In time clauses introduced by amikor (“when”), Hungarian normally uses the present tense even for future time:

amikor hazaérsz = “when you get home” (future sense from context) You can say amikor majd hazaérsz or even amikor haza fogsz érni, but the plain present is most natural.

What’s the difference between amikor and mikor here?

Use amikor to introduce a subordinate time clause (“when …”). mikor is the question word (“when?”) and is also common in informal speech as a subordinator, but amikor is the standard choice in writing: …, amikor hazaérsz.

Why hazaérsz and not haza jössz or otthon leszel?

hazaérsz focuses on the moment of arriving home (“get home/arrive home”).
haza jössz (“come home”) emphasizes the motion; it’s fine but can feel more about the process.
otthon leszel = “you will be at home,” i.e., the state of already being there. All are grammatical; choose based on nuance. For “when you get home (arrive),” hazaérsz is the crispest.

Is saying both felhívlak and telefonon redundant?

A bit, but it’s very common and natural. Felhív already implies “by phone,” yet telefonon can:

contrast with other channels (chat, video call, knocking), or
occupy the focus slot: Telefonon hívlak fel, nem videón.

How can I mark the future more explicitly?

Three natural options:

Plain present with context: Felhívlak (majd), amikor hazaérsz.
With the particle majd (“later/then”): Majd felhívlak, amikor hazaérsz.
With the auxiliary fog: Fel foglak hívni, amikor hazaérsz. All are correct; the plain present is very common.

Can I add akkor after the if-clause?

Yes. Ha muszáj, akkor felhívlak… mirrors English “If … then …” and is perfectly idiomatic. It doesn’t change the meaning, just adds a slight organizing emphasis.

Why is it hívlak, not hívlek?

Standard Hungarian uses hívlak for “I call you.” You may hear hívlek in some informal speech, but it’s nonstandard. Treat hívlak as the correct form to learn.

How would negation or a question change the word order?

The preverb moves after the verb:

Negation: Ha nem muszáj, nem hívlak fel.
Question: Ha muszáj, mikor hívlak fel? This shift is automatic with negation, focus, many questions, and some commands.