Learn Hardcore Finnish: Tämä maanantai on tavallinen päivä.

Tämä means “this” and is directly modifying maanantai (Monday), so tämä maanantai literally means “this Monday” as a specific day.

Tämä maanantai on tavallinen päivä.
→ This Monday is an ordinary day. (Talking about a particular Monday.)
Tänään on maanantai.
→ Today is Monday. (Talking about what weekday it is today.)

So:

tämä maanantai = “this Monday” (a specific Monday in a sequence of Mondays)
tänään on maanantai = “it is Monday today” (today’s weekday)

They are related in meaning but not interchangeable. The first one treats maanantai as the subject (this Monday), the second one uses tänään (“today”) as a time adverbial.

Why is maanantai in the basic form and not maanantaina? I learned that maanantaina means “on Monday”.

You’re right that maanantaina means “on Monday” (adessive case), but here maanantai is not an adverbial (“on Monday”) – it is the subject of the sentence.

Tämä maanantai = this Monday (subject in nominative case)
on tavallinen päivä = is an ordinary day

If you say:

Tänä maanantaina on tavallinen päivä.
→ literally “On this Monday there is an ordinary day.”

then tänä maanantaina (“on this Monday”) is an adverbial of time, and päivä becomes the subject. In the original sentence, maanantai itself is the day you’re talking about, so it stays in the nominative maanantai.

Why do we say on tavallinen päivä and not just on tavallinen? Is tämä maanantai on tavallinen also correct?

Both are grammatically correct, but the nuance is slightly different.

Tämä maanantai on tavallinen päivä.
→ This Monday is an ordinary day. (Explicitly calls it a day.)
Tämä maanantai on tavallinen.
→ This Monday is ordinary. (Describes Monday as ordinary, but doesn’t repeat the word “day”.)

In Finnish, olla + adjective without a noun is fine, just like “This Monday is ordinary” in English.
Adding päivä makes the sentence feel a bit more concrete and closer to “an ordinary day”, which mirrors the English more closely.

Why are tämä, maanantai, tavallinen, and päivä all in the basic (nominative) form?

There are two structures here:

Tämä maanantai
- tämä (this) modifies maanantai (Monday)
- both are in nominative because together they form the subject.
tavallinen päivä
- tavallinen (ordinary) modifies päivä (day)
- both are also in nominative because this is the predicative (what the subject “is”).

In sentences with olla (“to be”) where you say “X is Y”:

the subject (X) is in nominative
the predicative (Y = noun/adjective describing X) is also in nominative

Examples:

Tämä talo on uusi. – This house is new.
Tämä maanantai on tavallinen päivä. – This Monday is an ordinary day.

You would use partitive instead only in special meanings (like “some kind of”, “partly”, or ongoing processes), which don’t apply here.

Can the word order be changed? For example, can I say Tavallinen päivä on tämä maanantai?

Yes, Finnish word order is relatively flexible, and you can say:

Tavallinen päivä on tämä maanantai.

This is grammatically correct but changes the focus:

Tämä maanantai on tavallinen päivä.
→ Neutral: you’re talking about this Monday and describing it.
Tavallinen päivä on tämä maanantai.
→ More contrastive: among the days you’re thinking about, this Monday is the one that is an ordinary day (implying that maybe other days are not).

Other possible (though less neutral) orders:

Tavallinen tämä maanantai on päivä. – odd and unnatural.
Tämä on tavallinen maanantai päivä. – ungrammatical; maanantai and päivä don’t combine like that.

The original word order is the most natural, neutral one.

How would I say “On this Monday, it is an ordinary day” instead of “This Monday is an ordinary day”?

To say “On this Monday” as a time expression, you change tämä maanantai into the adessive case tänä maanantaina:

Tänä maanantaina on tavallinen päivä.

Literally:

tänä maanantaina = on this Monday
on tavallinen päivä = there is an ordinary day / it is an ordinary day

So:

Tämä maanantai on tavallinen päivä.
→ This Monday is an ordinary day. (Monday = subject)
Tänä maanantaina on tavallinen päivä.
→ On this Monday, it is an ordinary day / there is an ordinary day. (this Monday = time adverbial)

Finnish doesn’t have “a/an” or “the”. How do I know that tavallinen päivä means “an ordinary day” and not “the ordinary day”?

Finnish has no articles, so tavallinen päivä can correspond to:

an ordinary day
the ordinary day
sometimes even just ordinary day (in a general sense)

Context decides which English article you use. In this sentence:

Tämä maanantai on tavallinen päivä.

we’re introducing this Monday as just one ordinary day among many, so “an ordinary day” is the most natural translation.

If the context already made it clear which specific “ordinary day” you mean, English might use “the ordinary day”, but Finnish would still say tavallinen päivä. The language doesn’t mark the difference grammatically.

Could you omit on here, like sometimes happens in colloquial speech?

In standard Finnish, you must include on in this sentence:

Tämä maanantai on tavallinen päivä. ✅
Tämä maanantai tavallinen päivä. ❌ (incorrect in standard language)

In very colloquial Finnish, the verb olla (“to be”) can sometimes be dropped in short answers or certain structures, but not in a full declarative sentence like this. So even in casual speech, you would normally keep on here.

How would I say similar sentences like “Mondays are ordinary days” or “This Monday is very ordinary”?

Here are some natural variants:

“Mondays are ordinary days.”
- Maanantait ovat tavallisia päiviä.
  - maanantait = Mondays (plural nominative)
  - ovat = are
  - tavallisia päiviä = ordinary days (plural partitive, because you’re talking about them in a general, non-exhaustive way)
“This Monday is very ordinary.”
- Tämä maanantai on hyvin tavallinen.
  - hyvin = very
  - adjective tavallinen used alone as predicative
Or, staying closer to the original structure:
- Tämä maanantai on hyvin tavallinen päivä.
  → This Monday is a very ordinary day.