Learn Hardcore Polish: Było mi wstyd, gdy zapomniałem o jej urodzinach. - I was ashamed when I forgot her birthday.

było – past tense, neuter, 3rd person of być (to be), used impersonally (like English “it was”).
mi – dative “to me” (the experiencer of the feeling).
wstyd – noun “shame”, used as the thing that “exists for me”.

Byłem wstyd is ungrammatical. If you want a personal form, you must use an adjective or participle:

Byłem zawstydzony. – I was ashamed / embarrassed. (speaker male)
Byłam zawstydzona. – (speaker female)

But Było mi wstyd is far more common and idiomatic in everyday speech.

What exactly does mi mean here, and why is it in this form, not mnie?

Mi is the dative form of ja (I), meaning “to me / for me”.

In this sentence:

Było mi wstyd = “It was shame *to me → I felt ashamed.”*

Polish often uses the dative case for the experiencer of a feeling or state:

Było mi zimno. – I was cold. (lit. It was cold to me.)
Jest mi smutno. – I’m sad. (lit. It is sad to me.)
Było jej przykro. – She felt sorry / bad.

About mi vs mnie:

mi is the short, unstressed (clitic) form, used in neutral sentences like here: Było mi wstyd.
mnie is the full, stressed form, used for emphasis or in some syntactic positions:
- Mnie było wstyd, a jemu nie. – I was ashamed, but he wasn’t.

In your sentence the neutral, natural choice is mi.

Could I also say Wstydziłem się, gdy zapomniałem o jej urodzinach? Does it mean the same thing?

Yes, Wstydziłem się, gdy zapomniałem o jej urodzinach is correct and very natural. It means almost the same thing:

Było mi wstyd… – more like “I felt ashamed / embarrassed” (state).
Wstydziłem się… – literally “I was ashamed / I felt shame / I was ashamed of myself” (more verbal).

Nuance:

Było mi wstyd is slightly more neutral and descriptive of your emotional state.
Wstydziłem się can sound a bit more active / reflective, as if you were consciously ashamed or reproaching yourself.

In everyday conversation, both are fine; the choice is stylistic. If the speaker is female, the verb form changes:

Wstydziłam się, gdy zapomniałam o jej urodzinach.

Why is there a comma before gdy (Było mi wstyd, gdy zapomniałem…)?

In Polish, you normally put a comma before conjunctions that introduce a subordinate clause, such as:

gdy, kiedy, ponieważ, że, chociaż, jeśli etc.

Here, gdy zapomniałem o jej urodzinach is a subordinate clause of time (“when I forgot about her birthday”), so you must write:

Było mi wstyd, gdy zapomniałem o jej urodzinach.

If you reverse the order, you also use a comma:

Gdy zapomniałem o jej urodzinach, było mi wstyd.

What’s the difference between gdy and kiedy here? Can I replace gdy with kiedy?

Yes, you can safely replace gdy with kiedy:

Było mi wstyd, kiedy zapomniałem o jej urodzinach.

Both gdy and kiedy can mean “when” (in the temporal sense) and are usually interchangeable in such sentences.

Subtle difference:

kiedy is more common in everyday spoken language.
gdy can sound a bit more formal, literary, or written, though it’s also used in speech.

In this specific sentence, both sound completely natural.

Why is it zapomniałem and not something else? How would it change if the speaker were a woman?

Zapomniałem is the past tense, 1st person singular, masculine form of zapomnieć (to forget).

Polish past tense endings show person + gender:

zapomniałem – I forgot (speaker = male)
zapomniałam – I forgot (speaker = female)

Other forms:

zapomnieliśmy – we (at least one man) forgot
zapomniałyśmy – we (all female group) forgot

So if the speaker is a woman, the sentence becomes:

Było mi wstyd, gdy zapomniałam o jej urodzinach.

Why is the verb perfective (zapomniałem) and not imperfective (zapominałem)?

Polish distinguishes aspect: perfective vs imperfective.

zapomnieć (perfective) → zapomniałem
Focus on a single, completed event: I (once) forgot.
zapominać (imperfective) → zapominałem
Focus on a process or repeated/habitual action: I was forgetting / I used to forget.

In your sentence you’re talking about one specific incident (her birthday on that particular date), so you use the perfective:

Było mi wstyd, gdy zapomniałem o jej urodzinach. – I was ashamed when I (once) forgot her birthday.

Zapominałem o jej urodzinach would suggest you repeatedly or habitually forgot her birthdays (I kept forgetting her birthdays), which is a different meaning.

Why is it o jej urodzinach and not just jej urodziny or na jej urodziny?

The verb zapomnieć normally takes the preposition o:

zapomnieć o kimś / o czymś – to forget about someone / something.

So you say:

zapomniałem o jej urodzinach – I forgot about her birthday.

Breaking it down:

o – preposition “about”, here used with the locative case.
(o) jej urodzinach – about her birthday (locative plural).

Alternatives mean something different:

jej urodziny without o would be a direct object pattern, which is less natural here (zapomnieć jej urodziny is possible but sounds less common / more bookish).
na jej urodziny means “for her birthday / to her birthday party”, e.g.
- Idę na jej urodziny. – I’m going to her birthday (party).
- Kupiłem coś na jej urodziny. – I bought something for her birthday.

With zapomnieć, the most idiomatic is zapomnieć o jej urodzinach.

Why is urodzinach in this strange plural form? Why not singular?

In Polish, urodziny (birthday) is a plural-only noun (pluralia tantum). It doesn’t normally have a singular form in this meaning.

Its basic forms are:

Nominative: urodziny – birthday
Genitive: urodzin
Dative: urodzinom
Accusative: urodziny
Instrumental: urodzinami
Locative: urodzinach

In your sentence we have o jej urodzinach:

o requires the locative case → urodzinach.
The noun is inherently plural, so all its cases are plural.

So you can’t say “o jej urodzinie” in standard Polish when you mean “her birthday”; you must use the plural form urodziny / urodzinach.

What case is urodzinach, and which prepositions take this case?

Urodzinach is locative plural (miejscownik liczby mnogiej).

The locative in modern Polish is almost always used only after certain prepositions, such as:

o – o kim? o czym? → o jej urodzinach (about her birthday), o tobie, o pracy
w – w kim? w czym? → w domu, w szkole
na – na kim? na czym? (in some meanings) → na stole, na imprezie
po – po kim? po czym? → po obiedzie, po ulicy
przy – przy kim? przy czym? → przy oknie, przy stole

So:

o jej urodzinach – about her birthday (locative after o).

Can I change the position of mi? For example, say Mi było wstyd or Było wstyd mi?

Yes, but word order affects emphasis.

Neutral, most natural:

Było mi wstyd.

Other variants:

Mi było wstyd. – puts strong emphasis on “me”: I was ashamed (maybe others weren’t). This is fine, but marked as contrastive.
Było wstyd mi. – possible, but sounds unusual / poetic / heavily emphasized, not something you’d normally say in everyday speech.

Because mi is usually unstressed, it tends to appear right after the verb in neutral sentences:

Było mi wstyd, gdy zapomniałem… – standard, idiomatic order.

Could I say Było mi wstyd zapomnieć o jej urodzinach instead of using gdy?

Yes, that sentence is also correct:

Było mi wstyd zapomnieć o jej urodzinach.

Here zapomnieć is an infinitive, working like “to forget”:

Literally: “It was shame to me to forget about her birthday.”

Nuance and style:

Było mi wstyd, gdy zapomniałem o jej urodzinach.
– Focus on the time/event when it happened; more narrative.
Było mi wstyd zapomnieć o jej urodzinach.
– Focus more on the action itself (forgetting) as the cause of shame; slightly more compact and can sound a bit more written.

Both are natural; the original with gdy has a clearer “when X happened” storyline.

Do I always have to use o after zapomnieć, or can I say zapomnieć coś?

You have a few patterns with zapomnieć, and more than one is correct:

zapomnieć o kimś / o czymś (with o
- locative) – very common, very natural
- zapomnieć o jej urodzinach – forget about her birthday
- zapomniałem o spotkaniu.
zapomnieć czegoś (with genitive) – also correct, a bit more formal/bookish in many contexts
- zapomnieć czyichś urodzin – forget someone’s birthday
- zapomniałem twojego nazwiska.
zapomnieć coś (with accusative) – heard in colloquial speech, sometimes treated as less careful/standard
- Zapomniałem PIN.
- In careful standard language, zapomnieć PIN-u (genitive) is often preferred.

For birthdays, the most natural, everyday choice is:

zapomnieć o czyichś urodzinach → zapomniałem o jej urodzinach.

Why do we use jej here and not ją? And could we say urodziny jej?

In o jej urodzinach, jej is a possessive pronoun meaning “her” (as in her birthday), not an object pronoun.

Forms:

jej – her as a possessive (before a noun):
- jej urodziny – her birthday
- jej dom – her house
ją – her as a direct object (accusative):
- Widzę ją. – I see her.
- Znam ją. – I know her.

In your sentence we need “her” as a determiner for urodziny, so it must be jej, not ją.

As for urodziny jej:

It is grammatically possible, but sounds marked, emphatic, or somewhat old-fashioned/poetic in modern Polish.
The normal, neutral word order is jej urodziny / o jej urodzinach, with the possessive before the noun.