Learn Hardcore Finnish: Kerrataan yhdessä historiaa ennen koetta. - Let’s review history together before the exam.

It is present passive/impersonal: kerrata → kerrataan.
The Finnish “passive” is often called impersonal: there is no explicit subject, and it can often be translated as English “we”, “one”, or “people in general”.
In context, Kerrataan yhdessä… is best translated as “Let’s review together…”.

So kerrataan = “(we) review / let’s review” in a general, subjectless form.

Why is there no subject pronoun like me (“we”) in this sentence?

Finnish usually does not need subject pronouns when the verb ending already tells you the subject.

Here, though, it’s not even a normal personal form. Kerrataan is passive/impersonal, which is always subjectless:

Personal: Me kertaamme historiaa. = “We are reviewing history.”
Impersonal: Kerrataan historiaa. = “Let’s review history.”

In the impersonal form, you simply don’t use me. The idea of “we” or “people in general” is built into the form kerrataan.

What is the function of yhdessä and what kind of word is it?

Yhdessä means “together”.

Grammatically:

It is an adverb, not a noun.
It tells how the action is done: review together.

So Kerrataan yhdessä historiaa… = “Let’s review history together…”

(For background: historically yhdessä is related to yksi “one” + an old case ending, but for learners it’s easiest just to remember it as the adverb “together”.)

Why is historiaa in the form historiaa and not historia or historian?

Historiaa is the partitive singular of historia (“history”).

In Finnish, the object of a verb can be:

Total object (often -n form): Kerrataan historia.
- Implies “we (fully) go through all of history” or a clearly bounded amount.
Partitive object (often -a/-ä): Kerrataan historiaa.
- Implies an unbounded amount or not necessarily complete: “we go over (some) history / the subject of history in general”.

In a school context, historiaa usually feels natural because you are reviewing the subject “history”, not literally the entire history of the universe from beginning to end. The partitive gives it that more general, “some / a bit of” nuance.

Is historiaa singular or plural?

Historiaa is singular, just in the partitive case.

Basic forms:

Nominative singular: historia
Partitive singular: historiaa
Nominative plural: historiat
Partitive plural: historioita

So in this sentence, we are talking about history as a school subject (singular), not “histories”.

What about koetta — what form is that, and why is it used after ennen?

Koetta is the partitive singular of koe (“test, exam”).

The word ennen (“before”) is a preposition that requires the partitive case:

ennen koetta = “before the test”
ennen tuntia = “before the lesson”
ennen lomaa = “before the holiday”

So the pattern is: ennen + [noun in partitive]. That’s why we get koetta, not koe or kokeen.

Could we change the word order, for example: Ennen koetta kerrataan yhdessä historiaa?

Yes. Finnish word order is fairly flexible, especially for time and place expressions.

Both are correct:

Kerrataan yhdessä historiaa ennen koetta.
Ennen koetta kerrataan yhdessä historiaa.

The difference is mainly emphasis:

Starting with Ennen koetta highlights the time frame (“Before the test, that’s when we’re doing this…”).
Ending with ennen koetta sounds more like a neutral, natural flow “…before the test.”

Grammatically, both are fine.

How would this sentence look without the impersonal form, using a regular “we” form?

You can express the same idea using the personal me form:

Me kertaamme historiaa ennen koetta.

This is present indicative, 1st person plural:

me = “we”
kertaamme = “(we) review”

Meaning-wise, Me kertaamme… and Kerrataan… are similar, but:

Kerrataan… sounds a bit more like a suggestion/invitation (“Let’s review…”).
Me kertaamme… can feel a bit more like a statement of fact (“We are reviewing / we review…”).

How would I say “You (singular) review history before the test” instead of “Let’s review history”?

Use the imperative 2nd person singular of kerrata:

Kertaa historiaa ennen koetta. = “Review history before the test.”

Differences:

Kerrataan yhdessä historiaa…
- Impersonal: “Let’s review history together before the test.”
- Includes the speaker.
Kertaa historiaa ennen koetta.
- Imperative to “you”: a command/advice.
- Does not include the speaker.

Is ennen koetta one fixed phrase, or can ennen be used in other positions?

Ennen always comes before the noun phrase it governs, and that noun phrase is in the partitive:

ennen koetta = before the test
ennen lounasta = before lunch
ennen joulua = before Christmas

The whole time expression ennen koetta can move around in the sentence:

Kerrataan yhdessä historiaa ennen koetta.
Ennen koetta kerrataan yhdessä historiaa.

But ennen itself does not move away from its noun; it stays attached to koetta as a unit.

Is this sentence more formal or informal, and in what situations would it be used?

Kerrataan yhdessä historiaa ennen koetta. is neutral in tone and perfectly natural in:

a classroom setting, e.g. a teacher speaking to students
a study group, students speaking to each other
written instructions or materials aimed at students

It is neither especially formal nor slangy. A teacher could easily say this to a class, and students could also say it among themselves.