Tämä hillo on makeaa, mutta ei liian makeaa aamupalaksi.

Because hillo is being treated as a mass noun / substance here, like jam as food, not as a countable object.

With mass nouns, Finnish often uses the partitive singular in this kind of predicate:

• Kahvi on kuumaa = The coffee is hot
• Ruoka on hyvää = The food is good
• Hillo on makeaa = The jam is sweet

So makeaa is the partitive singular form of makea.

Yes, you may hear that too, but makeaa is the more natural choice in this kind of food-description sentence.

A rough way to think about the difference is:

• makeaa = talking about the jam as a substance you taste/eat
• makea = more like giving it a general property or classification

The difference is not always huge in everyday speech, but for foods and drinks, the partitive predicate is very common. So as a learner, Tämä hillo on makeaa is a very safe and natural pattern.

For the same reason: it is still describing hillo in the same predicate structure.

The second half is basically understood as:

• mutta ei ole liian makeaa or
• mutta se ei ole liian makeaa

Since the jam is still being described as a substance, the adjective stays in the partitive singular: makeaa.

Also, liian does not change the case. It just modifies the adjective:

• liian makeaa = too sweet

Because Finnish often leaves out repeated words when they are easy to understand from context.

The full version would be something like:

• Tämä hillo on makeaa, mutta ei ole liian makeaa aamupalaksi.
• Tämä hillo on makeaa, mutta se ei ole liian makeaa aamupalaksi.

In the original sentence, ole is omitted because the meaning is already clear from the first clause. This kind of ellipsis is very normal and natural.

Liian means too or excessively.

It is an adverb, and adverbs do not agree with nouns in case or number. That is why liian stays the same.

Examples:

• liian kuumaa = too hot
• liian suolaista = too salty
• liian makeaa = too sweet

The adjective changes form when needed, but liian does not.

Aamupalaksi means for breakfast or as something to have for breakfast.

The ending -ksi is the translative case. One of its common uses is to show:

• purpose
• role
• use
• suitability

So here:

• liian makeaa aamupalaksi = too sweet for breakfast

You will see the same pattern in other expressions:

• jälkiruoaksi = for dessert / as dessert
• lahjaksi = as a gift
• iltapalaksi = for an evening snack

So this is not really about movement or change here; it is about what something is suitable for.

They both mean breakfast, but there is a slight difference in feel.

• aamupala = very common everyday word
• aamiainen = also correct, but can sound a bit more formal or a bit more like hotel breakfast / buffet breakfast in some contexts

So aamupalaksi is very natural in this sentence.

Because Finnish does not have articles like a or the.

Instead, Finnish uses context or words like:

• tämä = this
• tuo = that
• se = it / that

So:

• Tämä hillo = this jam

If the sentence only said Hillo on makeaa, English might translate it as:

• Jam is sweet
• The jam is sweet
• Some jam is sweet

The exact English article depends on context, but Finnish does not need one.

Finnish word order is fairly flexible, but the original order is very natural.

The sentence is built like this:

• Tämä hillo = subject
• on makeaa = is sweet
• mutta ei liian makeaa = but not too sweet
• aamupalaksi = for breakfast

Putting aamupalaksi at the end works well because it tells us in what respect the jam is not too sweet.

You could rearrange things in some contexts, but the original version is a smooth, neutral way to say it.