Let's have a look at the preposition su. Its most common meaning in English is "on." As Marika has been explaining in video lessons such as this one, the simple preposition su can be combined with a definite article — in Italian, there are several forms, based on gender and quantity — to become a preposizione articolata (a preposition combined with a definite article — ("the" in English).
So to say, "on the table," instead of saying su il tavolo, we say sul tavolo. The preposition and definite article combine into one word.
Aspettate, lascio il libro sul tavolo
Wait, I'll leave the book on the table
Caption 3, Corso di italiano con Daniela - Il futuro
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This process is similar for all the different forms of definite articles in Italian.
sul = su + il
sull' = su + l'
...nubi invece sull'Umbria e sulle zone interne della Toscana.
...clouds, instead, in Umbria and in the inland areas of Tuscany.
Caption 63, Anna e Marika - in TG Yabla Italia e Meteo - Part 2
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sulla = su + la
Allora, siamo qui con la nostra? -Chiara.
So, we're here with our... -Chiara.
Che ci risponderà a un po' di domande sulla mozzarella di bufala.
Who will answer a few of our questions about buffalo mozzarella.
Captions 1-2, Anna e Marika - La mozzarella di bufala - La produzione e i tagli - Part 2
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sullo = su + lo
Sullo sfondo potete vedere il Vesuvio
In the background, you can see Vesuvius
Caption 4, Escursioni Campane - Castello Normanno - Part 1
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sui = su + i
Allora, questa lista la scriviamo tutti insieme,
So, this list we'll all write together,
io alla lavagna e voi sui quaderni.
I on the blackboard and you in your notebooks.
Captions 10-11, Corso di italiano con Daniela - Il condizionale - Part 5
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sugli = su + gli
Just as in English, prepositions often have multiple meanings and su is no exception.
Su can mean "on," but also "in," sometimes:
L'ho letto sul giornale.
I read about it in the newspaper.
Caption 22, Adriano Olivetti - La forza di un sogno Ep.2 - Part 10
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Su often means "about."
E vi racconto qualche storia semplice sul gelato, ma molto interessante.
And I'll tell you a few simple stories about ice cream, but very interesting.
Caption 10, Andromeda - in - Storia del gelato - Part 1
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Note that sometimes definite articles are used in Italian but not in English, as in the example above.
Su can mean "out of," as in the following example:
Nove volte su dieci lo fa perché ha qualcosa da nascondere.
Nine times out of ten, he does it because he has something to hide.
Caption 25, Provaci Ancora Prof! - S1E2 - Un amore pericoloso - Part 19
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A good expression to know is sul serio (seriously)?
Sul serio?
Seriously?
Caption 4, Marika spiega - La formazione degli aggettivi
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It can also be interpreted as "for real."
Però voglio dirti una cosa, questa è importante sul serio.
But I want to tell you something. This is important for real.
Caption 45, Francesca - Cavalli - Part 1
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Another way we use the preposition su is to give an approximate time, weight, or age.
Arriverò sul presto (I'll get there on the early side).
Aveva sui cinquant'anni (he was around fifty years old).
Note that in this lesson, we talked about the preposition su, but su is also an adverb meaning up, upwards. We'll talk about that in a future lesson.
Maybe you have seen or heard other uses of su we didn't mention here. Let us know!