Learn Hardcore Finnish: Tämä uusi muki on suuri, joten kahvia ei tarvitse kaataa toista kertaa. - This new mug is large, so there is no need to pour coffee a second time.

Tämä uusi muki = This new mug (subject; nominative)
Tämän uuden mukin would be the genitive (possessive or “of this new mug”), e.g.:
- Pidän tämän uuden mukin väristä. = I like the color of this new mug.

In Tämä uusi muki on suuri, the structure is:

Tämä – demonstrative pronoun, nominative singular
uusi – adjective, nominative singular
muki – noun, nominative singular (head of the noun phrase)

So: "This new mug is big." → the whole noun phrase is in the nominative, so tämä, not tämän.

Why is uusi in that form? Why not uuden mukin or uusi muki something else?

Uusi matches muki in case and number because it’s an adjective describing the noun:

Subject noun: muki → nominative singular
Adjective: uusi → also nominative singular

They agree: uusi muki = new mug.

You’d use uuden mukin (both in genitive) when the noun phrase itself is in genitive, for example:

Näen uuden mukin. – I see the new mug. (here uuden mukin is object in genitive)
Tämän uuden mukin korva on iso. – The handle of this new mug is big.

In your sentence, the noun phrase is the subject, so nominative uusi muki is correct.

What is the difference between suuri and iso? Could I say Tämä uusi muki on iso instead?

Both suuri and iso mean big, and in this sentence they are interchangeable:

Tämä uusi muki on suuri…
Tämä uusi muki on iso…

Subtle differences:

suuri is a bit more formal or bookish, often used in writing, in abstract meanings (suuri ilo = great joy) and in more official contexts.
iso is more colloquial / everyday, very common in spoken Finnish.

So Tämä uusi muki on iso would sound very natural in everyday speech. The meaning doesn’t change here.

How does joten work here? Is it like “so”, “therefore”, or “because”?

Joten is a conjunction that means roughly “so / therefore / thus”, introducing a consequence.

Structure:

Tämä uusi muki on suuri, joten kahvia ei tarvitse kaataa toista kertaa.
→ This new mug is big, so you don’t need to pour coffee a second time.

Important points:

The cause: Tämä uusi muki on suuri
The result: kahvia ei tarvitse kaataa toista kertaa
Punctuation: a comma before joten is standard.

You would not translate joten as because; because corresponds to koska:

Kahvia ei tarvitse kaataa toista kertaa, koska tämä uusi muki on suuri.
= You don’t need to pour coffee a second time, because this new mug is big.

So:

joten = so / therefore (result)
koska = because (reason)

Why is it kahvia and not kahvi? What is the partitive doing here?

Kahvia is the partitive form of kahvi (coffee). Partitive is used for:

An indefinite or uncountable amount of something:
- Juon kahvia. – I drink (some) coffee.
- Ostatko maitoa? – Are you buying (some) milk?
Often after negation, especially with “not having / not needing / not doing some amount of something”.

In this sentence:

kahvia ei tarvitse kaataa
literally: “coffee does not-need-to-be-poured” → you don’t need to pour (any) coffee.

The idea is: not a specific, countable “one coffee”, but coffee as a substance / some amount of coffee. So the partitive kahvia is natural.

If you said kahvi ei tarvitse kaataa, it would be ungrammatical and also conceptually strange: we don’t talk about “one whole coffee” as a concrete object in that way in Finnish; we talk about some coffee, more coffee etc., which favors the partitive.

Why is there no subject in ei tarvitse kaataa? Who “does not need to pour”?

Finnish often uses impersonal constructions where English would say “you” or “we”.

Ei tarvitse kaataa literally: “(it) is not needed to pour” / “no need to pour”.

The implied subject is generic:

“you” in general,
or “we”,
or “one” (like French on).

So, natural English translations:

…so you don’t need to pour coffee a second time.
…so there’s no need to pour coffee a second time.

If you did want an explicit subject, you could say:

Minun ei tarvitse kaataa kahvia toista kertaa. – I don’t need to pour coffee a second time.
Meidän ei tarvitse kaataa kahvia toista kertaa. – We don’t need to pour coffee a second time.

But the original sentence uses the impersonal “no need to” style, very common in Finnish.

Why is it ei tarvitse and not ei tarvitseka or something with -ko/-kö?

Two separate points:

Negative verb + main verb
- Finnish forms negation with a separate negative verb ei plus the main verb in a special form.
- For “need / have to” in the 3rd person singular:
  - Affirmative: (se) tarvitsee – it needs
  - Negative: (se) ei tarvitse – it doesn’t need
In the impersonal “no need” structure, we just say ei tarvitse without a stated subject.
-ko / -kö is a question particle, not part of negation:
- Tarvitseeko kaataa? – Do (you) need to pour?
- Eikö tarvitse kaataa? – Don’t (you) need to pour?

In your sentence, it’s a statement, not a question, so no -ko/-kö:

ei tarvitse kaataa – (one) does not need to pour.

Why is the verb after tarvitse in the form kaataa? Could it be kaatamaan or kaadettaa?

After tarvita in the meaning “to need to / have to (do something)”, Finnish usually uses the basic infinitive (1st infinitive) of the verb:

tarvitsee kaataa – needs to pour
ei tarvitse kaataa – does not need to pour

So kaataa is the basic dictionary form of the verb (1st infinitive).

Other forms have different meanings:

kaatamaan (illative of the 3rd infinitive) is used with motion verbs etc.:
- Menin keittiöön kaatamaan kahvia. – I went to the kitchen to pour coffee.
kaadattaa is a causative (“to have something poured / make someone pour”), a different verb.

So in this construction:

(ei) tarvitse + 1st infinitive is the normal pattern.
Therefore: ei tarvitse kaataa, not ei tarvitse kaatamaan.

What exactly does toista kertaa mean, and why are both words in that form (partitive)?

Toista kertaa literally means “a second time”.

Forms:

toinen = second (ordinal number)
- toista = its partitive form
kerta = time, occasion
- kertaa = partitive singular

So:

toista kertaa = “(for) a second time / another time / again (once more)”.

Why partitive?

Toista – as part of a time expression, it behaves like a partitive of ordinal number, commonly used in phrases like:
- toista päivää – for the second day (in a row)
- toista viikkoa – for the second week
Kertaa – time expressions with kertaa (once, twice, three times, etc.) are usually in the partitive in these “how many times (repeated)” expressions when combined with verbs of doing, especially in contexts like not needing to do again.

So the whole phrase kaataa toista kertaa = “to pour (for) a second time”.

A close paraphrase is:

…joten kahvia ei tarvitse kaataa uudelleen. – so you don’t need to pour coffee again.

Could I say uudestaan or uudelleen instead of toista kertaa?

Yes. All of these are natural, with slightly different nuance:

toista kertaa – literally “a second time”; emphasizes number of times.
uudestaan – again, anew; very common in speech.
uudelleen – again; a bit more neutral/formal than uudestaan.

Examples:

…joten kahvia ei tarvitse kaataa toista kertaa.
…joten kahvia ei tarvitse kaataa uudestaan.
…joten kahvia ei tarvitse kaataa uudelleen.

All mean essentially: “…so you don’t need to pour coffee again.”

What is the difference between muki and kuppi? Would Tämä uusi kuppi on suuri… also work?

Both refer to drinking vessels, but with different typical shapes:

muki = mug
- Usually taller, often with straight sides, usually handled, bigger volume (for coffee, tea, etc.).
kuppi = cup
- Typically smaller, more like a teacup or coffee cup, often part of a cup-and-saucer set; also used in some fixed expressions.

In this context:

Tämä uusi muki on suuri… – This new mug is big…
Tämä uusi kuppi on suuri… – This new cup is big…

Both are grammatically fine; the choice expresses what kind of thing it is: a mug or a cup. Because the idea is that it holds a lot of coffee, muki feels especially natural here, but kuppi is not wrong.

Could I change the word order, for example: Kahvia ei tarvitse kaataa toista kertaa, joten tämä uusi muki on suuri?

Yes, that sentence is grammatically correct, but the emphasis changes.

Original:
Tämä uusi muki on suuri, joten kahvia ei tarvitse kaataa toista kertaa.
→ Focus: The mug is big, therefore no need to pour again. The “bigness of the mug” is more foregrounded.
Reversed:
Kahvia ei tarvitse kaataa toista kertaa, joten tämä uusi muki on suuri.
→ Focus: The fact that you don’t need to pour again is stated first; then the size of the mug is presented as a kind of conclusion or result. In real life, you would usually first notice the mug is big, then conclude that you don’t need to refill, so the original order feels more natural.

Finnish word order is fairly flexible, but the information structure (what is new/important) shifts when you move clauses around. In everyday conversation, the original version is more typical.