Kahvin lisäksi juon vettä aamulla.

It means besides coffee, in addition to coffee, or as well as coffee.

In this sentence, lisäksi is working like a postposition: it comes after the noun phrase it belongs to. So Finnish says literally something like coffee's in-addition-to.

Because lisäksi normally takes the genitive form of a noun.

• kahvi = coffee
• kahvin = of coffee / coffee's

So:

• kahvin lisäksi = besides coffee

This is something you often just have to learn together with the postposition: lisäksi + genitive.

Juoda is the infinitive form, meaning to drink.

Juon is the 1st person singular present tense form, meaning I drink.

A few forms of juoda are:

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

So in the sentence, juon is the correct finite verb form.

Because Finnish often leaves out subject pronouns when they are clear from the verb ending.

Here, juon already tells you the subject is I.

So:

• minä juon = I drink
• juon = I drink

Both are possible, but omitting minä is very normal unless you want extra emphasis.

Because vettä is the partitive form of vesi.

• vesi = water
• vettä = water (partitive)

After verbs like juoda when you mean some amount of something, Finnish often uses the partitive object.

So:

• juon vettä = I drink water / I drink some water

This is especially common with uncountable substances like water, milk, coffee, tea, etc.

Juon veden would sound more like I drink the water or I drink up the water.

That uses a total object, which suggests the action is directed at a specific whole amount.

Compare:

• juon vettä = I drink water / some water
• juon veden = I drink the water / all the water

So vettä is the natural choice in your sentence.

Because Finnish often uses the adessive case (-lla / -llä) for times of day.

• aamu = morning
• aamulla = in the morning

Similar expressions include:

• päivällä = during the day
• illalla = in the evening
• yöllä = at night

So aamulla is a very normal time expression.

It is the present tense, and in Finnish the present tense can cover both meanings depending on context.

So juon can mean:

• I drink
• I am drinking

In this sentence, because of aamulla (in the morning), it most naturally sounds like a habitual statement:

• I drink water in the morning

No, Finnish word order is fairly flexible because the case endings show how words relate to each other.

Your sentence:

• Kahvin lisäksi juon vettä aamulla.

is natural and puts emphasis on besides coffee.

Other possible orders include:

• Juon vettä aamulla kahvin lisäksi.
• Aamulla juon vettä kahvin lisäksi.
• Juon aamulla vettä kahvin lisäksi.

These all mean roughly the same thing, but the focus and emphasis change.

Starting with Kahvin lisäksi highlights that idea first. It sets up the sentence as:

• As for what I drink besides coffee...

So the speaker is emphasizing the addition to coffee.

If the sentence started with Juon, it would sound a bit more neutral. By putting kahvin lisäksi first, Finnish gives that part extra prominence.

Sometimes, but it would not mean exactly the same thing.

• myös = also / too
• lisäksi = in addition / besides

Compare:

• Juon myös vettä aamulla. = I also drink water in the morning.
• Kahvin lisäksi juon vettä aamulla. = Besides coffee, I drink water in the morning.

The sentence with myös does not explicitly say in addition to coffee. It just says water is also something I drink.

It is singular partitive.

Even though English sometimes thinks of substances as a general mass, Finnish usually keeps words like water in the singular when talking about an unspecified amount:

• vettä = water / some water

A plural would be unusual here. So vettä does not mean waters; it is simply the normal way to express an uncountable substance as an object.