Learn Hardcore Finnish: Koulussa oli tänään paloharjoitus, ja palohälytys soi kovaa. - Today there was a fire drill at school, and the fire alarm rang loudly.

In this sentence, Koulussa oli tänään paloharjoitus, the point is that the fire drill took place in the school / at school, so the inessive is used. Finnish often uses the inessive for locations like “at school”, “at work”, etc. (e.g. töissä = at work).

Why is it “Koulussa oli tänään paloharjoitus” instead of something like “Tänään oli paloharjoitus koulussa”?

Both word orders are possible, but they have slightly different emphasis.

Koulussa oli tänään paloharjoitus.
→ Emphasis on where something happened: At school, there was a fire drill today.
Tänään oli koulussa paloharjoitus.
→ Emphasis on when: Today, there was a fire drill at school.

Finnish is quite flexible with word order; important information is often moved toward the beginning of the sentence. The version you have is a typical existential sentence:
(Location) + oli + (something that existed there)
→ Koulussa oli paloharjoitus. = There was a fire drill at school.

What is the function of oli here? Why isn’t there an equivalent of “there” like in “There was a fire drill”?

Finnish doesn’t use a word like English “there” in existential sentences. Instead, it uses this pattern:

[Place in a local case] + oli + [thing that exists there]

So:

Koulussa oli paloharjoitus.
Literally: In the school was a fire drill.
Natural English: There was a fire drill at school.

The verb oli is the past tense of olla (to be). Its role here is the same as “was” in English, but the location (koulussa) replaces “there”.

What exactly is paloharjoitus, and how is it formed?

Paloharjoitus is a compound noun:

palo = fire
harjoitus = exercise, practice, drill

Put together: paloharjoitus = fire drill (a practice for what to do if there is a fire).

Compounds like this are extremely common in Finnish; the first part specifies the type, and the last part tells you what it is (here, a harjoitus).

And what about palohälytys? How is that different from paloharjoitus?

Again, a compound:

palo = fire
hälytys = alarm, alert

So palohälytys = fire alarm.

Difference:

paloharjoitus = the event / drill
palohälytys = the alarm itself (the sound and/or the alarm system)

In the sentence:
Koulussa oli tänään paloharjoitus, ja palohälytys soi kovaa.
→ There was a fire drill, and the fire alarm rang loudly.

What does soi mean here, and what verb does it come from?

Soi comes from the verb soida = to ring, to sound, to play (a sound).

Forms relevant here:

(se) soi – present: it rings / it is ringing
(se) soi – past: it rang

The 3rd person singular past tense of soida is also soi, identical in form to the present. Context and other past-tense elements (like oli, tänään in a narrative of past events) make it clear that this is past:
→ palohälytys soi = the fire alarm rang.

Why is it kovaa and not something like kovasti or lujaa?

Kovaa, kovasti, and lujaa can all be translated as “loudly / hard / strongly”, but they’re used slightly differently.

kova = hard, loud (adjective)
kovaa = partitive form of kova, often used as an adverb → loudly, hard
kovasti = an adverb, often strongly / a lot / very much
lujaa = “hard, fast, loud” (quite colloquial in some uses)

In the context of sounds, soida kovaa is very natural and common:

Palohälytys soi kovaa. = The fire alarm rang loudly.

Kovaa here is functioning as an adverb of manner, formed from the adjective kova in the partitive. This is a common adverb-formation pattern in Finnish (e.g. hiljaa from hiljainen).

What case is kovaa, and why is that case used for an adverb?

Kovaa is in the partitive singular of kova.

In Finnish, many adverbs of manner come directly from adjectives in the partitive singular. Examples:

kova → kovaa (hard, loud → loudly, hard)
hiljainen → hiljaa (quiet → quietly)
nopea → nopeasti (here with -sti)

So while English often adds -ly (loud → loudly), Finnish often uses:

adjective in partitive (e.g. kovaa, hiljaa)
or an adverb-forming suffix like -sti (e.g. nopeasti).

In soi kovaa, kovaa just acts as an adverb = “loudly”.

Why is there a comma before ja in “… paloharjoitus, ja palohälytys soi kovaa”?

Finnish comma rules differ a bit from English. In Finnish, you generally do use a comma before ja when it connects two independent clauses (each with its own subject and verb):

Koulussa oli tänään paloharjoitus,
ja palohälytys soi kovaa.

Each part could stand as its own sentence:

Koulussa oli tänään paloharjoitus.
Palohälytys soi kovaa.

So a comma before ja is appropriate here. In English, we may use a comma before and in such cases, but it’s more optional and style-dependent.

Finnish doesn’t have articles, but how do I know that paloharjoitus means “a fire drill” and not “the fire drill”?

You infer “a” vs “the” from context, word order, and whether the thing is already known.

In Koulussa oli tänään paloharjoitus:

We are introducing this event as new information, so natural English uses “a fire drill”.

If it had been previously mentioned or clearly known, English might use “the fire drill”, but Finnish would usually still just say paloharjoitus. If you really need to specify definiteness, you add other words, e.g.:

se paloharjoitus = that fire drill
tämänpäiväinen paloharjoitus = today’s fire drill

But plain paloharjoitus itself is neutral; English must choose a or the based on context.

Why is koulussa used instead of something like koululla? What’s the difference?

Both are locative cases but with different nuances:

koulussa (inessive: in/inside the school)
– Usually means inside the building or in the context of school (“at school”).
koululla (adessive: at/on/by the school)
– Often used for being at the premises or in the yard / area around the school, or at an institution as a place of activity.

In everyday speech, “at school” is almost always koulussa. So:

Koulussa oli tänään paloharjoitus.
→ Sounds like the normal way to say “There was a fire drill at school today.”

Koululla oli paloharjoitus could work in some contexts, but it would feel like you’re emphasizing the location as a site/premises, not just “at school” in the general sense.

How should I pronounce paloharjoitus and palohälytys? Where is the stress?

In Finnish, stress is always on the first syllable of a word.

paloharjoitus → PA-lo-har-joi-tus
- Stress on PA
- harjoitus breaks as har-jo-i-tus; oi is a diphthong (one syllable)
palohälytys → PA-lo-hä-ly-tys
- Stress on PA
- Each vowel is its own sound (no diphthongs here)

Pronunciation tips:

Every written vowel is pronounced.
No reduction like English “schwa”; vowels stay clear.
Consonants are pronounced as written and are usually short here (no doubled consonants in these words).

Could I say “Palohälytys soi tänään kovaa koulussa” instead? Would that still be correct?

Yes, that sentence is grammatically correct, but the focus changes:

Koulussa oli tänään paloharjoitus, ja palohälytys soi kovaa.
→ First tells you what happened at school, then describes the alarm.
Palohälytys soi tänään kovaa koulussa.
→ Focuses directly on the alarm and when/where it rang.

Finnish allows a lot of word-order flexibility as long as the cases and verb forms are correct. However, the original sentence nicely presents:

The event (paloharjoitus)
A detail about it (palohälytys soi kovaa) in a natural narrative order.