Aamulla juon mieluummin mehua kuin kahvia.

Aamulla is the form Finnish uses for in the morning. The basic noun aamu means morning, but Finnish often uses a case ending for time expressions.

So:

• aamu = morning
• aamulla = in the morning

This is a very common pattern with parts of the day:

• aamulla = in the morning
• päivällä = in the daytime / during the day
• illalla = in the evening
• yöllä = at night

Finnish usually leaves out the subject pronoun when it is already clear from the verb ending.

Here, juon means I drink, because the -n ending marks first person singular.

So both are possible:

• Juon = I drink
• Minä juon = I drink

Using minä is not wrong, but it often adds emphasis, contrast, or extra clarity. In a neutral sentence, leaving it out is more natural.

Juon is the 1st person singular present tense form of the verb juoda (to drink).

A few present-tense forms 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 juon by itself already tells you the subject is I.

Mieluummin means rather, preferably, or more willingly.

In this sentence, it shows preference:

• juon mieluummin mehua kuin kahvia = I drink juice rather than coffee / I prefer to drink juice instead of coffee

Its position is natural because it comes right before the thing being preferred: mehua.

A very common pattern is:

• mieluummin X kuin Y = rather X than Y

For example:

• Syön mieluummin kalaa kuin lihaa. = I’d rather eat fish than meat.

They are in the partitive form. This is extremely common after verbs like juoda when talking about an unspecified amount of a substance.

So:

• mehu = juice
• mehua = some juice / juice
• kahvi = coffee
• kahvia = some coffee / coffee

In Finnish, when you drink or eat some amount of something, especially a mass noun like juice, coffee, water, tea, bread, etc., the partitive is usually used.

So:

• Juon mehua. = I drink juice.
• Juon kahvia. = I drink coffee.

This does not sound strange in Finnish; it is the normal choice here.

Kuin is used in comparisons, and in this sentence it means than.

So:

• mieluummin mehua kuin kahvia = rather juice than coffee

This is a standard structure in Finnish:

• mieluummin X kuin Y = rather X than Y

You also see kuin after comparative forms:

• isompi kuin = bigger than
• parempi kuin = better than

So here, kuin connects the preferred option (mehua) with the less preferred one (kahvia).

It can mean either, depending on context.

Aamulla can mean:

• in the morning in general
• this morning in a specific context

And Finnish present tense is often used for both:

• general habits
• near-present or context-based statements

So this sentence could mean:

• a general preference in the mornings
• a statement about what the speaker is doing this morning

If you want to make it clearly habitual, Finnish often uses aamuisin:

• Aamuisin juon mieluummin mehua kuin kahvia. = In the mornings, I’d rather drink juice than coffee.

This is a very useful distinction.

• aamulla = in the morning / this morning
• aamuisin = in the mornings, regularly

So:

• Aamulla juon mieluummin mehua kuin kahvia.
  This can be general or context-specific.

• Aamuisin juon mieluummin mehua kuin kahvia.
  This more clearly means a repeated habit: In the mornings, I prefer to drink juice rather than coffee.

If you want to stress routine, aamuisin is often the better choice.

Yes. Finnish word order is flexible, though not completely free. Changing the order usually changes emphasis, not the basic meaning.

For example:

• Aamulla juon mieluummin mehua kuin kahvia.
  Neutral, with in the morning as the starting frame.

• Juon aamulla mieluummin mehua kuin kahvia.
  Also natural; starts with the verb.

• Minä juon aamulla mieluummin mehua kuin kahvia.
  Adds emphasis to I.

The original sentence starts with Aamulla because Finnish often likes to put time or place information first.

Finnish has no articles, so it does not have direct equivalents of English a/an and the.

That means:

• mehua can mean juice, some juice, or in context the juice
• kahvia can mean coffee, some coffee, or in context the coffee

The exact meaning comes from context, not from an article.

This is one reason why case endings, context, and word choice are very important in Finnish.

Because both mehua and kahvia are objects of the same kind in the comparison, and both fit the same grammar: they are both things being drunk in an unspecified amount, so both are in the partitive.

So Finnish keeps the structure parallel:

• mehua = juice
• kahvia = coffee

This is very natural in comparisons:

• Juon mieluummin teetä kuin kahvia.
• Syön mieluummin omenaa kuin banaania.

The two compared nouns often appear in matching forms when they play the same role in the sentence.