Learn Hardcore Finnish: Minä juon kahvia naapurin kanssa.

You’re talking about an unspecified amount of coffee, not one clearly defined “unit”.
Verbs like juoda (“to drink”) very often take the partitive when you just mean “(some) coffee” in general.

So:

Minä juon kahvia. = I drink (some) coffee. / I’m drinking coffee.
Minä juon kahvin. = I drink the coffee / I finish the (whole) coffee.

The basic dictionary form is kahvi, but in this context Finnish prefers kahvia.

What case is kahvia, exactly, and how do I recognize it?

Kahvia is partitive singular.

You can recognize it by:

Nominative singular (dictionary form): kahvi
Partitive singular ending: often -a/-ä or -ta/-tä

So:

kahvi → kahvia

Partitive is extremely common, especially:

With indefinite amounts (coffee, water, bread)
With ongoing or incomplete actions
With many verbs like juoda (to drink), syödä (to eat), ostaa (to buy) when the amount is not specified.

Why is it Minä juon and not just Juon? Do I always need the word minä?

You don’t always need minä.

The verb ending in Finnish already shows the person:

juon = “I drink / I am drinking”

So:

Minä juon kahvia naapurin kanssa.
Juon kahvia naapurin kanssa.

Both mean the same thing: I drink / I am drinking coffee with the neighbor.

Using minä often adds a bit of emphasis (like “I am drinking coffee with the neighbor.”) or is used for clarity, but grammatically you can leave it out.

What is the dictionary form of juon, and how is it formed?

The dictionary (infinitive) form is juoda (“to drink”).

Juon is the 1st person singular present tense:

Take juoda
Drop -da → juo-
Add the personal ending -n (for “I”) → juon

A small pattern to remember:

syödä (to eat) → syön
juoda (to drink) → juon
voida (to be able to) → voin

So the -da/-dä infinitive type often makes the “I”-form by dropping -da/-dä and adding -n.

Why is it naapurin kanssa and not naapuri kanssa?

With kanssa (“with”), Finnish uses the genitive case:

Nominative: naapuri (“neighbor”)
Genitive: naapurin

The pattern is: [genitive] + kanssa = “with [someone/something]”

So:

naapurin kanssa = with (the/my) neighbor
ystävän kanssa = with (a/the/my) friend
lapsen kanssa = with (the/my) child

Naapuri kanssa is ungrammatical; you must use naapurin before kanssa.

What case is naapurin?

Naapurin is genitive singular of naapuri (“neighbor”).

The genitive often answers English “whose?” or is used in many set constructions, including with kanssa:

naapurin auto = the neighbor’s car
naapurin kanssa = with the neighbor

Formally:

Nominative: naapuri
Genitive: naapurin

Does naapurin kanssa mean “with my neighbor” or “with the neighbor” or “with a neighbor”?

Finnish has no articles (“a”, “an”, “the”), so naapurin kanssa can mean:

with my neighbor
with the neighbor
with a neighbor

The exact nuance is decided by context. In everyday speech, if you say:

Juon kahvia naapurin kanssa.

people will usually understand it as “I’m drinking coffee with my neighbor”, unless there’s a reason to think otherwise.

Can I change the word order, like Juon kahvia minä naapurin kanssa or Kahvia juon minä naapurin kanssa?

Finnish word order is flexible, but there is a neutral/default order:

Minä juon kahvia naapurin kanssa.
(Or without minä:) Juon kahvia naapurin kanssa.

You can move words around for emphasis or style, but not every order sounds natural. Some possibilities:

Kahvia juon naapurin kanssa.
- Emphasis on kahvia: It’s coffee (not something else) that I drink with the neighbor.
Naapurin kanssa juon kahvia.
- Emphasis on naapurin kanssa: It’s with the neighbor (not alone / not with someone else) that I drink coffee.

But something like Juon kahvia minä naapurin kanssa is unusual and often sounds awkward in ordinary speech. In general, keep:

Verb near the beginning
Subject (if used) early
Other elements after, unless you have a reason to emphasize them.

How would the sentence change if I wanted to say “I drink the (whole) coffee with the neighbor”?

You would likely use kahvin (accusative/genitive) instead of kahvia:

Minä juon kahvin naapurin kanssa.

This suggests a specific, complete unit of coffee, e.g. a cup of coffee that you finish:

juon kahvia = I drink (some) coffee / I’m drinking coffee
juon kahvin = I drink / finish the coffee (the whole portion, cup, etc.)

Context still matters, but kahvin gives more of a “complete object” feeling.

How do I say this sentence in the negative: “I don’t drink coffee with the neighbor”?

Finnish uses a special negative verb ei plus the main verb stem:

Positive: Minä juon kahvia naapurin kanssa.
Negative: Minä en juo kahvia naapurin kanssa.

Breakdown:

en = “I don’t”
juo = negative form of juoda for “I”
kahvia naapurin kanssa stays the same

You can also drop minä:

En juo kahvia naapurin kanssa.

What is the basic word-by-word meaning of each word?

Minä = I
juon = (I) drink / am drinking
kahvia = coffee (partitive: some coffee / coffee as an indefinite amount)
naapurin = (of) the neighbor / the neighbor’s (genitive)
kanssa = with

Together: “I drink (some) coffee with the neighbor.”

How do I pronounce juon and kahvia?

Rough pronunciation (using English approximations):

juon ≈ “yoo-on” said smoothly as one syllable, like YOO-on (Finnish actually treats it as one syllable with a diphthong [uo])
- j like English “y” in “yes”
- u like “oo” in “food”
- o like “o” in “soft” (British-ish)
- n as in English
kahvia ≈ “KAH-via”
- k as in “cat”
- a like “a” in “father”
- h pronounced clearly (no silent h)
- v like English “v”
- i like “ee” in “see”
- a again like “father”

Stress is always on the first syllable in Finnish:

JUON
KAH-via
NAA-pu-rin (for naapurin)

If I want to say “I’m drinking coffee with my neighbors” (plural), how does naapuri change?

You need the plural genitive of naapuri:

Nominative plural: naapurit (“neighbors”)
Genitive plural: naapurien (standard) or naapureiden (very common)

With kanssa, use the genitive plural:

Minä juon kahvia naapurien kanssa.
Minä juon kahvia naapureiden kanssa.

Both mean: I drink / I’m drinking coffee with my neighbors. The form naapureiden is very common in everyday speech.