Learn Hardcore Finnish: Aamulla juon kahvia mukista ja katson järvelle. - In the morning I drink coffee from a mug and look at the lake.

Aamu is the basic form (nominative), meaning "morning."
Aamulla is the adessive case form and is used for many time expressions to mean "in the/at (the) X".

So:

aamu = morning (as a bare noun)
aamulla = in the morning, at morning time

Other common time expressions with -lla/-llä:

yöllä – at night
päivällä – in the daytime
illalla – in the evening
keväällä – in the spring

So Aamulla juon… literally is something like “On the morning I drink…”, but idiomatically “In the morning I drink…”.

Could you also say aamuisin instead of aamulla? What’s the difference?

Yes, you could say:

Aamuisin juon kahvia…

Aamulla focuses on a particular morning / mornings as a time of day.
Aamuisin is a frequentative time adverb and means “in the mornings / every morning / usually in the morning.”

Nuance:

Aamulla juon kahvia… – Talking about this morning, or mornings in general as a time frame.
Aamuisin juon kahvia… – Emphasizes habit: “I usually / always drink coffee in the morning.”

Both are grammatical; the choice depends on whether you want to stress habit or just the time of day.

Why is there no word for “I”? Where is the subject?

The subject “I” is included in the verb ending in Finnish.

juon = I drink
stem: juo- (drink)
-n ending = 1st person singular (“I”)

Similarly:

katson = I look / I watch
stem: katso-
-n = “I”

Because the verb ending already shows the person, the pronoun minä (“I”) is usually dropped unless you want to emphasize it:

Minä juon kahvia… – I drink coffee… (maybe in contrast to someone else)
Juon kahvia… – I drink coffee… (neutral)

So the Finnish sentence contains the subject implicitly.

Why is it kahvia and not kahvi after juon?

Kahvia is the partitive case of kahvi (“coffee”). Finnish often uses the partitive for:

Unspecified amount of a substance (mass noun)
- juon kahvia = I drink (some) coffee
- syön leipää = I eat (some) bread
Ongoing / not total action (“imperfective” nuance)

If you said:

Juon kahvin.

then kahvin is the accusative (same form as genitive here), and it means:

“I drink the coffee / I drink up a (whole) coffee (cup).”

So:

Juon kahvia. – I (am) drinking coffee / I drink coffee (in general, some amount).
Juon kahvin. – I drink (finish) that specific coffee.

In your sentence, we’re just talking about drinking some coffee in the morning, so kahvia is natural.

What exactly does mukista mean, and why do we use the -sta ending?

Muki = mug.
Mukista is the elative case (from inside something):

muki → mukista = from (out of) the mug

The verb juoda (“to drink”) very often takes an elative with containers:

juon kahvia mukista – I drink coffee from a mug
juon vettä pullosta – I drink water from a bottle
juon teetä kupista – I drink tea from a cup

So the -sta/-stä ending shows the direction “out of / from inside” the container, which is why mukista is used.

Is there any difference between muki and kuppi here?

Yes, but it’s mostly about what kind of vessel you imagine:

muki – a mug, usually taller, with a handle, often used for coffee/tea
kuppi – a cup, can be a smaller coffee/tea cup, part of a cup-and-saucer set, etc.

So you could also say:

Aamulla juon kahvia kupista… – In the morning I drink coffee from a cup…

Grammatically it works the same way: you’d still use elative:

mukista, kupista, lasista, pullosta etc.

Why is it järvelle and not järveä or järvellä?

Järvi = lake.

Different case forms:

järveä – partitive
järvellä – adessive (“on the lake / at the lake”)
järvelle – allative (“onto / to the lake, toward the lake”)

With katsoa (“to look”), Finnish can use different cases depending on what you’re expressing:

katson järveä – I look at the lake (as an object; focusing on the lake itself)
katson järvelle – I look toward the lake / out over the lake / in the direction of the lake

In your sentence, katson järvelle emphasizes the direction of the gaze, not just the lake as an object. It suggests looking out towards the lake, maybe from a window or shore.

So is katson järveä also correct, and how is it different from katson järvelle?

Both are correct, but the nuance changes:

Katson järveä.
- Focus: the lake itself as the thing you’re looking at.
- Neutral “I’m looking at the lake.”
Katson järvelle.
- Focus: the direction of your gaze, towards the area of the lake / lake landscape.
- Often feels a bit more scenic / spatial: looking out over the lake, toward the lake.

In everyday speech, people often prefer järveä if they mean simply “looking at the lake.” Järvelle is used when you imagine your eyes moving or pointing toward that area.

Could the word order be different, like Juon aamulla kahvia mukista ja katson järvelle? Is that still correct?

Yes, Finnish word order is fairly flexible. All of these are grammatical:

Aamulla juon kahvia mukista ja katson järvelle.
Juon aamulla kahvia mukista ja katson järvelle.
Juon kahvia aamulla mukista ja katson järvelle.

The differences are in emphasis and what comes first in the sentence:

Putting Aamulla at the beginning (as in your sentence) highlights the time: As for the morning, what I do is…
Starting with Juon puts more focus on the action.

But in normal, neutral speech, all these versions would be easily understood and acceptable.

Why isn’t there a comma before ja in …mukista ja katson järvelle like in English sometimes?

Finnish punctuation rules are a bit stricter about not using a comma in simple two-part clauses joined by ja (“and”).

In English, you might sometimes see a comma before “and,” especially in longer sentences. In Finnish:

If you have two verbs with the same subject and they’re joined by ja, you normally do not put a comma:
- Juon kahvia mukista ja katson järvelle.

You would use a comma if you were joining two independent clauses that are more separate, or if other conjunctions are involved, but here no comma is the standard.

How do we know if it means “from a mug” or “from the mug” when there are no articles like “a/the” in Finnish?

Finnish has no articles (no “a/an” or “the”), so the noun itself doesn’t show definiteness. Mukista can mean:

“from a mug”
“from the mug”

Which one is intended depends on context, for example:

If earlier you said:
- Tässä on muki. Aamulla juon kahvia mukista.
  → “Here is the mug. In the morning I drink coffee from the mug.”
If there’s no previous mention, an English speaker will often translate it as “from a mug”, since nothing makes it specific.

So Finnish leaves definiteness mostly implicit, and context fills in whether it’s specific or not.