Learn Hardcore Polish: On boi się, że rozbije telefon. - He is afraid that he will break the phone.

Się here is a reflexive particle, but in many verbs of emotion (like bać się, martwić się) it does not directly mean himself / herself in a literal way.

In English we say be afraid, but in Polish you say bać się (literally something like fear oneself historically, but synchronically it’s just part of the verb).
You should learn bać się as a fixed verb that always takes się in this meaning.
You do not translate się separately into English here.

So:
On boi się → He is afraid, not He fears himself.

Can the word order change? Is On się boi, że rozbije telefon also correct, and is there any difference?

Yes, both are correct:

On boi się, że rozbije telefon.
On się boi, że rozbije telefon.

They mean the same thing: He is afraid that he will break the phone.

Subtle points:

Both word orders are completely natural.
Very often speakers put się after the verb (boi się), but On się boi is also common.
Sometimes On się boi can sound slightly more expressive or emotional in context, but in isolation there is no real difference.

In everyday speech, you might even drop On and say:

Boi się, że rozbije telefon. – He is afraid that he will break the phone. (subject understood from context)

Why is there a comma before że in boi się, że rozbije telefon?

In Polish, you place a comma before conjunctions like:

że – that
gdy – when
ponieważ – because
chociaż – although

when they introduce a subordinate clause.

Here:

Main clause: On boi się – He is afraid
Subordinate clause: że rozbije telefon – that he will break the phone

Because że introduces the subordinate clause, Polish spelling rules require a comma:

On boi się, że rozbije telefon.

In English we can often drop that (“He’s afraid he’ll break the phone”). Can we drop że in Polish?

No, in this kind of sentence że is not normally dropped.

English: He is afraid (that) he will break the phone. – that can be omitted.
Polish: On boi się, że rozbije telefon. – you must keep że.

A version without że (e.g. On boi się rozbije telefon) is incorrect.

Why is it rozbije and not something like będzie rozbijał for the future?

Polish has two aspects: perfective and imperfective.

rozbić (pf.) – to break, to smash (a single, completed act)
rozbijać (impf.) – to be breaking repeatedly / to be in the process of breaking

Perfective verbs like rozbić form the simple future with present endings:

ja rozbiję – I will break
ty rozbijesz – you will break
on rozbije – he will break

So rozbije already means will break – a single future event.

Będzie rozbijał would use the imperfective and means something like:

He will be breaking (it, repeatedly / for some time).

That’s not what you want here. You mean one accidental future event, so rozbije is exactly right.

What is the difference between rozbije, rozbija, and rozbił?

They come from two different aspects:

rozbić (perfective) – single, completed act
- rozbiję – I will break
- rozbijesz – you will break
- rozbije – he/she/it will break
rozbijać (imperfective) – ongoing or repeated action
- rozbijam – I am breaking / I break (regularly)
- rozbijasz – you are breaking / you break
- rozbija – he/she/it is breaking / breaks

Past tense forms (from rozbić):

rozbił – he broke (once, completed)
rozbiła – she broke

So:

rozbije telefon – he will (once) break the phone.
rozbija telefon – he is breaking the phone / he breaks the phone (habitually).
rozbił telefon – he broke the phone.

Why not rozbija here? How would On boi się, że rozbija telefon sound?

On boi się, że rozbija telefon is grammatically possible but sounds odd in most contexts.

rozbije (perfective) → one future event:
On boi się, że rozbije telefon.
He is afraid that he will (once) break the phone.
rozbija (imperfective, present) would normally be:
- He is breaking the phone (right now).
- He breaks the phone (regularly).

So On boi się, że rozbija telefon would suggest something like:

He is afraid that he is (currently) breaking the phone / that he keeps breaking the phone (habitually).

That’s not the usual meaning “He’s worried he might drop it and break it once,” so speakers use rozbije.

Why is it telefon and not telefonu? Doesn’t bać się take genitive?

Two different verbs and two different objects are involved:

bać się + GENITIVE
- On boi się telefonu. – He is afraid of the phone.
  (Here telefonu is genitive, governed by bać się.)
rozbić + ACCUSATIVE
- On rozbije telefon. – He will break the phone.
  (Here telefon is accusative, the direct object of rozbije.)

In your sentence:

bać się doesn’t have a direct noun object; instead it has a clause: że rozbije telefon.
Inside that clause, rozbije has its direct object telefon in accusative.

So:
On boi się, że rozbije telefon.

boi się → no direct noun, only the że-clause
rozbije → takes telefon (accusative)

Why don’t we say jego telefon (“his phone”)? How do we know whose phone it is?

Polish often leaves possessives implicit when the owner is obvious from context, especially when it’s the subject of the sentence.

On boi się, że rozbije telefon.
Literally: He is afraid that he will break (the) phone.
Default interpretation: his own phone.

If you need to make it explicit, you can say:

On boi się, że rozbije swój telefon. – He is afraid that he will break his (own) phone.
On boi się, że rozbije jego telefon. – He is afraid that he will break his phone (someone else’s male person’s phone; can sound contrastive: not his own).

Swój normally refers back to the subject’s own possession, so it’s the most natural explicit version here.

Can I omit On and just say Boi się, że rozbije telefon?

Yes.

Polish allows subject pronouns to be dropped when they’re clear from the verb ending.

On boi się, że rozbije telefon. – full form
Boi się, że rozbije telefon. – natural, everyday form

Both mean He is afraid that he will break the phone, assuming context makes it clear we’re talking about “he” and not “she” or someone else.

Is there a more “noun-like” way to say this, using something like rozbicie?

Yes, you can nominalize the verb:

On boi się rozbicia telefonu.
Literally: He is afraid of the breaking of the phone.

Here:

rozbicia – verbal noun (gerund-like form) of rozbić, in genitive
telefonu – also in genitive, dependent on rozbicia

This structure is grammatical but sounds more formal or abstract.
In normal speech, On boi się, że rozbije telefon is much more natural.

How do you pronounce boi się and rozbije?

Approximate pronunciation (using English-friendly hints):

boi się → [BOH-yee shyeh]
- boi: two syllables, bo-i, like “bo”
  - “ee” (but smoother: [bɔ.i])
- się: sounds like shyeh ([ɕɛ]) – soft “sh” + “eh”
rozbije → [roz-BEE-yeh]
- roz: like roz in “Roswell” but with a clear z
- bi: bee
- je: yeh

Stress in Polish is almost always on the second-to-last syllable:

BOI się (BOI is the stressed syllable)
roz-BI-je (BI is stressed)