Learn Hardcore Finnish: Minä juon liikaa kahvia iltaisin. - I drink too much coffee in the evenings.

adds a bit of emphasis to I (as opposed to someone else)
can sound slightly more explicit or contrastive: Minä juon liikaa kahvia, mutta sinä juot liian vähän.

What is the base form of juon, and how is it conjugated?

Juon comes from the verb juoda (to drink).

Present tense of juoda:

minä juon – I drink
sinä juot – you drink (singular)
hän juo – he / she drinks
me juomme – we drink
te juotte – you drink (plural / formal)
he juovat – they drink

There is no separate auxiliary like do in Finnish. You just use the conjugated verb: juon, not teen juon or similar.

Why is it kahvia and not kahvi?

Kahvia is the partitive singular of kahvi (coffee). Finnish uses the partitive in several situations; here, the key idea is:

You are talking about an indefinite amount of a mass/uncountable noun (coffee as a substance), not a clearly delimited portion like “a cup of coffee”.

So:

Juon kahvia. – I drink coffee (some coffee, in general)
Juon liikaa kahvia. – I drink too much coffee.

If you were referring to specific units or a whole definite thing, other forms could appear, e.g.:

Juon tämän kahvin. – I drink this coffee (this particular one).
Juon kahvit. – I drink the coffees / have the coffees (e.g. the ordered coffees).

Does liikaa kahvia always require the partitive case?

With liikaa (too much), the noun almost always appears in the partitive:

liikaa kahvia – too much coffee
liikaa sokeria – too much sugar
liikaa töitä – too much work
liikaa ihmisiä – too many people

You can treat liikaa + noun in partitive as the normal fixed pattern. So yes, you should expect the partitive after liikaa in sentences like this.

What exactly does liikaa mean, and how is it different from liian paljon?

Both liikaa and liian paljon can mean too much.

Minä juon liikaa kahvia.
Minä juon liian paljon kahvia.

In everyday speech, these are almost interchangeable.

Nuances:

liikaa is a single word and very common; it feels a bit more compact and colloquial.
liian paljon literally means too much / too many and can sound slightly more explicit about “quantity”.

You can use either; for a learner, it is safe to treat them as synonyms in this context.

Why is kahvia in the middle of the phrase liikaa kahvia and not before liikaa?

The usual word order with liikaa is:

liikaa + [noun in partitive]

So:

liikaa kahvia – too much coffee
not
kahvia liikaa (though this can appear with a special emphasis on too much).

In kahvia liikaa, you would typically be stressing the excessiveness more strongly, often with intonation:

Minä juon kahvia liikaa. – I drink coffee too much (the focus is on “too much”, not so much on what you drink).

The neutral, unmarked way to say it is liikaa kahvia.

Can I change the word order, for example to Iltaisin juon liikaa kahvia?

Yes. Finnish word order is relatively flexible, and you can move parts for emphasis.

All of these are grammatical:

Minä juon liikaa kahvia iltaisin. – neutral; mild emphasis on “I”.
Juon liikaa kahvia iltaisin. – neutral, very typical.
Iltaisin juon liikaa kahvia. – emphasizes the time: In the evenings I drink too much coffee.
Liikaa kahvia juon iltaisin. – highlights how much coffee is the problem.
Kahvia juon liikaa iltaisin. – emphasizes that it is coffee (not something else) that you drink too much.

Meaning stays basically the same; the difference is nuance and focus.

Why is the present tense juon used, even though in English the meaning is habitual?

Finnish present tense covers several English uses:

simple present: I drink
present continuous: I am drinking
generic or habitual: I (usually) drink / I tend to drink
future meaning, depending on context.

So Minä juon liikaa kahvia iltaisin is naturally understood as a habitual statement because of iltaisin (“in the evenings (in general)”), even though grammatically it is just present tense. No separate “habitual tense” is needed.

What does iltaisin literally mean, and how is it formed?

Iltaisin is an adverb that means in the evenings, in the evening(s) habitually.

It is formed from:

ilta = evening
a temporal suffix -isin → iltaisin

This -isin suffix is used to form adverbs meaning “on Xs / in the Xs (repeatedly, habitually)”, for example:

aamuisin – in the mornings
päivisin – in the daytime / during the days
öisin – at night(s)
maanantaisin – on Mondays (regularly)

You can think of iltaisin as a single vocabulary item meaning “in the evenings (as a routine)”, rather than trying to parse it word-for-word.

What is the difference between iltaisin and illalla?

Both relate to the evening, but they differ in habitual vs specific time:

iltaisin = in the evenings (regularly, usually)
- Minä juon teetä iltaisin. – I drink tea in the evenings (as a habit).
illalla = in the evening, this evening, on that evening (usually a more specific time)
- Minä juon teetä illalla. – I will drink tea in the evening / tonight.

In your sentence:

Minä juon liikaa kahvia iltaisin.
→ suggests a repeated pattern: evenings in general.

Is Minä juon liikaa kahvia iltaisin formal, informal, or neutral?

It is neutral standard Finnish. You can use it:

in written Finnish (textbooks, emails, essays)
in spoken Finnish if you stick to standard language.

In everyday casual speech, many Finns would say something like:

Mä juon liikaa kahvii iltasin.

Changes in colloquial speech:

minä → mä
kahvia → kahvii
iltaisin → iltasin

As a learner, using the standard form Minä juon liikaa kahvia iltaisin is always safe and correct.

How would I say I have been drinking too much coffee in the evenings in Finnish?

The closest natural version uses the perfect tense of juoda:

Olen juonut liikaa kahvia iltaisin.

Breakdown:

olen – I am / I have (auxiliary for perfect)
juonut – past participle of juoda (drunk)
liikaa kahvia – too much coffee
iltaisin – in the evenings (habitually)

This corresponds to English I have drunk / I have been drinking too much coffee in the evenings. Finnish does not have a special continuous form like have been drinking; the perfect covers both meanings.

How can I emphasize that the problem is specifically the coffee, not something else?

In Finnish, emphasis is often done by word order and intonation. To highlight coffee as the problematic thing, you can move kahvia earlier:

Kahvia minä juon liikaa iltaisin.
→ As for coffee, I drink too much in the evenings.
Kahvia juon minä liikaa iltaisin.
→ Also strongly emphasizes kahvia (and the subject minä).

You can also contrast explicitly:

Kahvia juon liikaa, mutta teetä liian vähän.
– I drink too much coffee, but too little tea.

The original sentence is neutral; changing the word order puts extra focus on the element you move toward the beginning.

How do you pronounce Minä juon liikaa kahvia iltaisin?

Very roughly in IPA:

Minä – [ˈminæ] (stress on the first syllable)
juon – [juon]
liikaa – [ˈliːkɑː] (long ii and long aa)
kahvia – [ˈkɑhviɑ]
iltaisin – [ˈiltɑisin]

Whole sentence:
Minä juon liikaa kahvia iltaisin. → [ˈminæ juon ˈliːkɑː ˈkɑhviɑ ˈiltɑisin]

Key points:

Stress is almost always on the first syllable of each word.
Double vowels (ii, aa) are held noticeably longer than single vowels.
Every letter is pronounced; there are no silent letters.