Learn Hardcore Polish: Dzisiejszy wykład o historii świata zaczyna się o dziesiątej. - Today’s lecture on the history of the world begins at ten.

dzisiejszy is an adjective meaning “today’s,” and it modifies the noun wykład (lecture). It agrees with wykład in gender (masculine), number (singular) and case (nominative). By contrast, dziś and dzisiaj are adverbs meaning “today” and cannot directly qualify a noun.

What case is historia in after the preposition o, and why?

The preposition o meaning “about” takes the locative case. Therefore historia (nominative) becomes historii (locative). So o historii literally means “about (the) history.”

Why is świata in the genitive case here?

świata is the genitive singular of świat (world). It marks possession or “of-ness” in the phrase historia świata (“history of the world”). Inside that noun phrase, świata remains in genitive even when historia shifts to locative under o.

Why does the verb appear as zaczyna się rather than just zaczyna?

zaczynać is a transitive verb (“to start something”). The reflexive form zaczynać się means “to begin” intransitively. Here wykład is the subject that begins itself, so you need the particle się:
• wykład zaczyna would mean “(someone) starts the lecture.”
• wykład zaczyna się means “the lecture starts.”

Why is time expressed as o dziesiątej? What case and form is dziesiątej?

When telling the hour in Polish you use o + an ordinal number in the locative case (the implied noun is godzina, which is feminine). The ordinal dziesiąty (“tenth”) becomes dziesiątej (feminine locative singular). So o dziesiątej means “at ten o’clock.”

Can you include godzina (“hour”) in that phrase? How?

Yes. If you want to mention the word godzina explicitly, you say o dziesiątej godzinie. Here both dziesiątej and godzinie are in the locative case after o.

Could I say wykład z historii świata instead of o historii świata? What’s the difference?

Yes, you can say wykład z historii świata.
• z + genitive (z historii) typically indicates the subject area or course (“a lecture in world history,” like one item in a curriculum).
• o + locative (o historii) emphasizes the topic or commentary of that specific lecture (“a lecture about world history”).

Where is the stress in “Dzisiejszy wykład o historii świata zaczyna się o dziesiątej”?

Polish words are almost always stressed on the penultimate (second-to-last) syllable. So you get:
• Dzi-SIEJ-szy
• VYK-ład
• hi-STO-rii
• ŚWIA-ta
• za-CZY-na
• sie
• o dzię-SIA-tej