Kun kadulla on jäätä, liukastun helposti, joten kävelen hitaasti.

When there is ice on the street, I slip easily, so I walk slowly.

Breakdown of Kun kadulla on jäätä, liukastun helposti, joten kävelen hitaasti.

• minä: I
• olla: to be
• kävellä: to walk
• kun: when

Questions & Answers about Kun kadulla on jäätä, liukastun helposti, joten kävelen hitaasti.

Why does the sentence start with Kun? Does it mean when or if here?

Kun can mean both when (time) and if (condition), depending on context.
In this sentence it’s basically conditional: When/if there is ice on the street, I slip easily, so I walk slowly.
If you want a more clearly “conditional” feel, jos (if) is also common: Jos kadulla on jäätä…

What case is kadulla, and why is it used?

Kadulla is adessive case (ending -lla/-llä). With locations it often means on/at a place.

• katu = street
• kadulla = on the street / out on the street

Why is it on jäätä and not on jää?

Because Finnish often uses the partitive for an indefinite amount/substance. Jäätä is the partitive of jää (ice).
Kadulla on jäätä = There is (some) ice on the street (ice as an uncountable substance).
Using jää would sound like a more specific “the ice” or a defined quantity in contexts where that makes sense, but here the natural choice is partitive.