Learn Hardcore Polish: Wczoraj mój brat zakochał się w dziewczynie z pracy. - Yesterday my brother fell in love with a girl from work.

There is no normal Polish verb zakochać without się.
*zakochał dziewczynę is ungrammatical; the verb cannot take a direct object like that. You must say zakochał się w dziewczynie.

Why is it zakochał się w dziewczynie (literally “fell in love in a girl”) instead of using a word that means “with”, like in English?

Different languages use different prepositions with the same idea.

In Polish, the fixed pattern is:

zakochać się w + [locative] = to fall in love with someone/something

So you always say, for example:

zakochać się w dziewczynie – fall in love with a girl
zakochać się w nim – fall in love with him
zakochać się w Warszawie – fall in love with Warsaw (as a city)

Even though English uses with, Polish uses w (in) + locative case.

What grammatical case is dziewczynie, and why do we use that form here?

Dziewczynie is the locative singular form of dziewczyna (girl).

The noun changes because the preposition w in this construction requires the locative:

base form (nominative): dziewczyna – a girl
locative singular: dziewczynie

Pattern (simplified for many feminine nouns ending in -a):

Nominative: -a → dziewczyna
Locative: -e / -ie → dziewczynie

So we say:

w dziewczynie – in a girl (here: with a girl, as the one he fell in love with)

What does z pracy literally mean, and which case is used in pracy?

Z pracy literally means “from work”.

Here z is the preposition “from/out of”, and it takes the genitive case:

base form (nominative): praca – work, job
genitive singular: pracy

So:

z pracy – from work (i.e. from his workplace)

Contrast this with:

w pracy – at work (w
- locative: w pracy)

In your sentence, dziewczynie z pracy is “a girl from work”, not “a girl at work”.

Why is it mój brat and not mojego brata?

Mój brat is in the nominative case, used for the subject of the sentence.

mój brat – my brother (subject; nominative)
mojego brata – my brother in genitive/accusative (used for objects, after some prepositions, etc.)

In the sentence:

Wczoraj mój brat zakochał się…
mój brat is doing the action → it must be nominative.

How does mój change with gender and number?

Mój is the possessive pronoun “my”. It changes form depending on the noun it modifies.

Nominative singular:

masculine: mój brat – my brother
feminine: moja siostra – my sister
neuter: moje dziecko – my child

Nominative plural:

masculine personal (group of at least one male person): moi bracia – my brothers
non‑masculine‑personal (things, mixed/female groups): moje książki – my books

In your sentence brat is masculine, so we use mój brat.

Why is there no equivalent of “a” or “the” in w dziewczynie z pracy?

Polish has no articles (no words like a, an, the).

Whether you mean a girl from work or the girl from work is understood from context and stress, not from a separate word. So:

w dziewczynie z pracy can mean
- in a girl from work (indefinite)
- or in the girl from work (definite)

Depending on the wider context, a translator will choose a or the in English.

Can we move wczoraj to another place in the sentence, like in English?

Yes, word order in Polish is relatively flexible. All of these are grammatically correct, with slightly different emphasis:

Wczoraj mój brat zakochał się w dziewczynie z pracy.
Mój brat wczoraj zakochał się w dziewczynie z pracy.
Mój brat zakochał się wczoraj w dziewczynie z pracy.

Typically, time expressions like wczoraj often appear near the beginning, but they don’t have to. What matters most is that the relationships between words (shown by case endings and prepositions) stay the same.

Can się move, for example to Mój brat się zakochał w dziewczynie z pracy?

Yes. Się is a clitic and normally comes directly before or after the verb. Both are natural:

Mój brat zakochał się w dziewczynie z pracy.
Mój brat się zakochał w dziewczynie z pracy.

You cannot put się at the very beginning of the sentence, and you don’t separate it far from its verb. So:

*Się mój brat zakochał… – incorrect
Mój brat wczoraj się zakochał… – correct
Wczoraj mój brat się zakochał… – correct

What tense and aspect is zakochał, and how would I say “was falling in love”?

Zakochał is:

past tense,
masculine singular,
perfective aspect (one complete event).

zakochał się = he fell in love (a single finished event).

The imperfective partner is zakochiwać się:

zakochiwał się – he was falling in love / he used to fall in love (ongoing or repeated process)

What is the difference between zakochać się w dziewczynie and kochać dziewczynę?

They describe different stages:

zakochać się w dziewczynie – to fall in love with a girl
- the moment or process of starting to love her
- perfective (zakochać się) or imperfective (zakochiwać się)
kochać dziewczynę – to love a girl
- a state: you already love her
- imperfective verb only

So in your sentence, zakochał się w dziewczynie focuses on the new feeling appearing yesterday.

Could we omit mój and just say Wczoraj brat zakochał się w dziewczynie z pracy?

Yes, grammatically you can drop mój:

Wczoraj brat zakochał się w dziewczynie z pracy.

This then simply means “Yesterday (the/a) brother fell in love with a girl from work.”

Whether listeners understand it as my brother, his brother, her brother, etc. depends on context. Polish does not always mark possession when it’s obvious (especially with family members and body parts), but in a standalone sentence, mój brat is clearer.