Learn Hardcore Finnish: Paketti putosi, koska hylly oli epävakaa. - The package fell because the shelf was unstable.

Paketti is the subject of the sentence (“The package fell…”). In Finnish, the subject of a finite verb normally appears in the nominative case. If it were an object, you’d see a different case (for example, partitive), but putosi (“fell”) is intransitive, so its actor stays in nominative.

Could I say Putosi paketti instead of Paketti putosi?

Grammatically, Finnish word order is quite flexible, so Putosi paketti is not strictly wrong – it would emphasize “paketti” as new information or surprise. However, the unmarked, neutral order for a main clause is Subject–Verb–Object/Complement (SVO). Since you’re simply stating a fact, Paketti putosi sounds more natural.

Why isn’t hylly in the elative (hyllyltä) to show “from the shelf”?

Here hylly is not indicating “from where” but is itself the subject of the subordinate clause introduced by koska (“because”). In “koska hylly oli epävakaa” the shelf “was unstable,” so it remains in the nominative case as the subject of oli. If you wanted to say “The package fell from the shelf,” you’d indeed use hyllyltä: Paketti putosi hyllyltä.

Why is the verb oli (past tense “was”) used in the subordinate clause instead of the present on (“is”)?

The main clause verb putosi is in the past tense (“fell”), so the explanatory clause normally matches that time frame: “because the shelf was unstable.” You could use present—Paketti putosi, koska hylly on epävakaa—if you want to emphasize that the shelf is still unstable now. But matching tenses is more common when explaining why something happened.

Is the comma before koska mandatory?

In Finnish, you usually separate a main clause and a subordinate clause with a comma. After koska it’s strongly recommended (and often required) to insert a comma:
Paketti putosi, koska hylly oli epävakaa.

Why use koska here? Could I use sillä or siksi instead?

koska = “because” is the standard causal conjunction, introducing a clause that explains why.
sillä also means “because” but is actually a postpositive conjunction (“..., sillä ...”), more colloquial or spoken. You could say:
Paketti putosi, sillä hylly oli epävakaa.
siksi että is more formal and wordy:
Paketti putosi siksi että hylly oli epävakaa.
Choose koska for clear, straightforward causal clauses.

What exactly does epävakaa mean, and how is it formed?

epävakaa means “unstable.” It’s built from the negative prefix epä- (un-/non-) + the adjective vakaa (stable). So you’re literally saying “un-stable.”

Why is epävakaa in the nominative case and not epävakaana (essive) or another case?

In Finnish, predicate adjectives that follow the verb olla (“to be”) take the nominative case to agree with the subject:
Hylly (nom.) oli epävakaa (nom.).
Using epävakaana (essive) would read “as unstable,” suggesting a temporary role rather than describing its inherent state.

Could I use epävarma instead of epävakaa? They both have the prefix epä-.

No – epävarma means “uncertain” or “unsure,” usually about decisions or feelings. epävakaa specifically means physically or structurally “unstable.” Since you’re talking about a shelf that wobbles, epävakaa is the correct choice.

What’s the difference between putosi and kaatui? Could I say Paketti kaatui?

putoa/putosi = “to fall/fell” (dropping straight down).
kaatua/kaatui = “to tip over/fall over” (like something toppling).
A package doesn’t “tip over” in the same way a person or chair might; it more simply “falls.” That’s why putosi is more natural here.