Learn Hardcore Finnish: Sovittu aika on huomenna iltapäivällä.

sovittu aika = a time that has been agreed on in advance, typically between people (e.g. an appointment time).
sopiva aika = a suitable or convenient time in general, from someone’s point of view.

So:

Sovittu aika on huomenna iltapäivällä.
= The previously arranged/confirmed time is tomorrow afternoon.
Sopiva aika olisi huomenna iltapäivällä.
= A suitable/convenient time would be tomorrow afternoon.

In this sentence, the context is an already arranged appointment, so sovittu is used.

Can I change the word order, for example: Huomenna iltapäivällä on sovittu aika? Does it change the meaning?

Yes, you can change the word order, and the basic meaning stays the same, but the focus changes slightly.

Sovittu aika on huomenna iltapäivällä.
- Neutral emphasis: you’re stating when the agreed time is.
- Topic: the agreed time.
- New information: tomorrow afternoon.
Huomenna iltapäivällä on sovittu aika.
- Emphasis on when: you’re talking about tomorrow afternoon and saying that there is an agreed time then.
- Topic: tomorrow afternoon (or the time frame).
- New information: that there is an agreed time then.

Both are correct, but the original version is the most natural if you’re simply answering the question “When is the agreed time?”

Why is it iltapäivällä with -llä, not something like iltapäivänä or just iltapäivä?

Finnish often uses the adessive case (-lla / -llä) for time expressions, especially with parts of the day:

aamulla = in the morning
päivällä = in the daytime
iltapäivällä = in the afternoon
illalla = in the evening
yöllä = at night

So iltapäivä (afternoon) → iltapäivällä (in the afternoon).

You could say iltapäivänä (essive case), but that would usually refer more to a particular day’s afternoon in a slightly more formal or specific way, and even then iltapäivällä is far more common in everyday speech for “in the afternoon”.

In short:

huomenna iltapäivällä = tomorrow in the afternoon (normal, natural)
huomenna iltapäivänä = also possible, but much less common, sounds more formal/unusual in normal speech.

What is the difference between huomenna iltapäivällä and huomisen iltapäivällä / iltapäivänä?

All relate to “tomorrow afternoon,” but they’re built differently.

huomenna iltapäivällä
- huomenna = tomorrow (adverb)
- iltapäivällä = in the afternoon
- Very natural and common: “tomorrow afternoon”.
huomisen iltapäivällä / iltapäivänä
- huomisen = of tomorrow (genitive of huomenen/huominen)
- huomisen iltapäivällä ≈ on tomorrow’s afternoon (in the afternoon of tomorrow)
- huomisen iltapäivänä ≈ on tomorrow’s afternoon (using essive)
- These forms are grammatically correct but are less common and more formal/literary. In everyday speech huomenna iltapäivällä is strongly preferred.

So, for normal spoken Finnish, stick with huomenna iltapäivällä.

Is huomenna an inflected form of huominen?

They are related, but not in a simple “one declension table” way.

huomenna is an adverb meaning tomorrow.
huominen is an adjective/noun meaning tomorrow’s / the future, the next day.

Some examples:

Tapaaminen on huomenna. = The meeting is tomorrow.
Huominen päivä on kiireinen. = Tomorrow’s day is busy. / Tomorrow will be a busy day.

You don’t normally “decline” huomenna; it is already a fixed adverbial form used to express time, like English “tomorrow”.

Why do we need on here? Can Finnish drop the verb “to be” in a sentence like this?

In this kind of sentence, you need the verb on (is).

The structure is:

Sovittu aika = subject (nominative)
on = verb “to be”
huomenna iltapäivällä = adverbial of time

Finnish can sometimes drop words that are understood from context, especially pronouns:

(Se) on huomenna. → Often just On huomenna. (It’s tomorrow.)

But in a full nominal sentence like:

Sovittu aika on huomenna iltapäivällä. you cannot normally omit on in standard language.
Without on, the sentence would sound broken or like a note with missing words.

Could you also say just Sovittu aika on huomenna? Does that mean the same thing?

Yes, Sovittu aika on huomenna is correct and natural. It means:

The agreed time is tomorrow.

The difference is:

Sovittu aika on huomenna. = The agreed time is tomorrow (no information about morning/afternoon/evening).
Sovittu aika on huomenna iltapäivällä. = The agreed time is tomorrow afternoon (more specific).

So the shorter version loses the detail that it’s in the afternoon.

How would I make this sentence plural, like “The agreed times are tomorrow afternoon”?

You need to put both the subject and the verb in the plural:

Singular: Sovittu aika on huomenna iltapäivällä.
= The agreed time is tomorrow afternoon.
Plural: Sovitut ajat ovat huomenna iltapäivällä.
= The agreed times are tomorrow afternoon.

Changes:

sovittu aika → sovitut ajat
- sovittu → sovitut (plural form of the participle/adjective)
- aika → ajat (plural of the noun)
on → ovat (3rd person plural of “to be”)

How do you pronounce Sovittu aika on huomenna iltapäivällä? Any tricky parts for an English speaker?

Approximate pronunciation (with syllable breaks and stress):

So-vit-tu ai-ka on huo-men-na il-ta-päi-väl-lä

Key points:

Primary stress always on the first syllable of each word:
- SOvittu, AIka, HUOmenna, ILtapäivällä.
Double consonants like tt, nn, ll are long:
- sovittu → hold the t a bit longer: so-vit-tu
- huomenna → long n: huo-men-na
- iltapäivällä → long l: il-ta-päi-väl-lä
ai in aika is like in English eye.
äi in iltapäivällä is like in English bad but with the mouth a bit more tense, followed by i.

Nothing is silent; every vowel and consonant is pronounced.