Learn Hardcore Finnish: Ensi maanantaina tarkistan aikataulun ennen kokousta. - Next Monday I will check the schedule before the meeting.

Because maanantaina is in the essive case (-na/-nä), which is commonly used to express when something happens with days/dates: on Monday / next Monday.
ensi maanantai (no -na) is the nominative form and would be used more like a noun phrase, e.g. Ensi maanantai on vapaapäivä. (Next Monday is a day off.)

What does ensi mean, and how is it different from seuraava?

ensi means next (the upcoming one) and is very common with time expressions: ensi viikolla, ensi vuonna, ensi maanantaina.
seuraava also means next/following, but it often feels a bit more “in a sequence” (the one after the current one), and it’s used widely too: seuraava pysäkki (the next stop), seuraava maanantai.
In everyday Finnish, ensi maanantaina is usually the most natural for next Monday.

What case is maanantaina, and how do I form it for weekdays?

maanantaina is essive. For weekdays, you typically add -na/-nä:

maanantai → maanantaina
tiistai → tiistaina
keskiviikko → keskiviikkona
torstai → torstaina
perjantai → perjantaina
lauantai → lauantaina
sunnuntai → sunnuntaina
Use -nä after front vowels (ä, ö, y), otherwise -na.

Why isn’t maanantaina capitalized like Monday in English?

In Finnish, weekdays and months are not capitalized unless they start a sentence. So maanantaina is normally lowercase.

Why is tarkistan in the present tense if the meaning is in the future?

Finnish often uses the present tense to talk about scheduled or intended future actions, especially when a time expression is present (like ensi maanantaina).
So tarkistan literally looks like I check, but it commonly means I will check in this context.

How do I know tarkistan means “I check” (1st person singular)? What is the dictionary form?

The dictionary form (infinitive) is tarkistaa (to check / to verify).
The ending -n marks 1st person singular in the present tense:

(minä) tarkistan = I check / I’ll check
Finnish often drops the subject pronoun minä because the verb ending already shows the person.

Why is the object aikataulun and not aikataulu or aikataulua?

aikataulun is the total object form (for singular, it usually looks like the genitive -n). It suggests the action is seen as complete/whole: check the (whole) schedule.
If you meant a more ongoing/partial idea (check some of the schedule / look over the schedule a bit), you could use the partitive: tarkistan aikataulua.

Is aikataulun “genitive” or “accusative”? I’ve heard Finnish has an accusative.

In practice, for a singular total object with most present-tense forms, Finnish uses the genitive-looking -n form: aikataulun. Many textbooks call this the genitive object (or treat it as how accusative is realized in Finnish).
What matters for you as a learner: in an affirmative sentence like this, a complete/total object is typically -n in the singular.

Why is it ennen kokousta (partitive), not something like ennen kokous?

The preposition/postposition ennen requires the partitive case.
So:

ennen kokousta = before the meeting
The base form is kokous (meeting), and its partitive singular is kokousta.

What happens to kokous when it’s inflected into kokousta? Why does the word change?

kokous is a word type that forms its stem with -ou- in many cases:

nominative: kokous
partitive: kokousta
genitive: kokouksen
inessive (“in”): kokouksessa
So the stem used for most endings is kokouk-, and the partitive ending -ta gives kokousta.

Is the word order fixed? Could I move parts of the sentence around?

The meaning stays largely the same, but word order changes emphasis. Common alternatives:

Tarkistan aikataulun ensi maanantaina ennen kokousta. (more neutral, focuses on the action first)
Ennen kokousta tarkistan aikataulun ensi maanantaina. (highlights before the meeting)
Putting Ensi maanantaina first strongly frames the sentence with the time.

Any pronunciation tips for this sentence?

Key points:

Stress is usually on the first syllable: EN-si MAA-nan-tai-na TAR-kis-tan AI-ka-tau-lun EN-nen KO-kous-ta
ai in aikataulu is a diphthong (like eye): ai-
ou in kokous is also a diphthong: ko-kous
Finnish spelling is very consistent: each letter is pronounced, and vowel length matters (though this sentence doesn’t have tricky double vowels/consonants).