Мне кажется, что баклажан вкуснее, когда в соусе есть немного петрушки.

Because казаться in this meaning works like it seems to me.

• мне is dative: to me
• кажется is 3rd person singular: seems

So Мне кажется, что... literally means To me, it seems that...

Russian usually does not say я кажусь for this meaning.
Я кажусь... means I seem... / I appear... to other people, for example:

• Я кажусь уставшим. = I seem tired.

But in your sentence, the idea is it seems to me that..., so мне кажется is the natural structure.

Because there is no stated subject like я doing the action. The expression мне кажется is an impersonal-style construction meaning it seems to me.

Russian commonly uses 3rd person singular neuter-style forms in expressions like this:

• Мне нравится... = I like... / literally is pleasing to me
• Мне хочется... = I want... / literally it is desired to me
• Мне кажется... = it seems to me

So кажется is fixed here because the whole expression behaves like it seems.

Что introduces a subordinate clause, like English that:

• Мне кажется, что баклажан вкуснее...
• It seems to me that eggplant is tastier...

In English, that is often optional, but in Russian что is very common and usually sounds natural in this kind of sentence.

The singular баклажан is being used in a general sense, like talking about eggplant as a food or ingredient in general.

Russian often uses the singular to talk about something as a category:

• Кофе полезен. = Coffee is good for you.
• Банан вкуснее с шоколадом. = Banana is tastier with chocolate.

So баклажан вкуснее means something like eggplant tastes better or eggplant is tastier in general, not necessarily one single physical eggplant.

Вкуснее is the normal comparative form of вкусный:

• вкусный = tasty
• вкуснее = tastier

Russian usually prefers this short comparative form rather than более вкусный, especially in everyday speech.

So:

• баклажан вкуснее = eggplant is tastier

Более вкусный is possible, but it is usually less natural here and can sound more formal or analytical.

In Russian, the comparative can stand on its own when the comparison is understood from context.

So баклажан вкуснее, когда... means something like:

• Eggplant tastes better when...
• Eggplant is tastier when...

The unstated comparison is something like than usual or than without parsley in the sauce.

This is very natural in Russian. English does the same thing:

• It’s better with lemon.
• Coffee tastes better hot.

Когда means when, and it introduces the condition or situation in which something is true:

• баклажан вкуснее, когда...
• eggplant is tastier when...

So the sentence means that the taste is better in the situation where the sauce has a little parsley.

Russian often uses когда in places where English might also say if in a looser, everyday way, but here when is the direct and natural meaning.

Because в meaning in takes the prepositional case when talking about location.

• соус = sauce
• в соусе = in the sauce

This is the regular pattern:

• в супе = in the soup
• в салате = in the salad
• в соусе = in the sauce

So в соусе есть... literally means there is ... in the sauce.

Russian есть has two different meanings:

1. to eat
2. there is / there are

Here it means there is:

• в соусе есть немного петрушки
• there is a little parsley in the sauce

This existential есть is very common when introducing the presence of something somewhere.

Compare:

• Я ем. = I am eating.
• На столе есть хлеб. = There is bread on the table.

So here it has nothing to do with eating.

Because немного is followed by the genitive case.

• петрушка = parsley
• genitive singular: петрушки

So:

• немного петрушки = a little parsley

This is a very common pattern in Russian:

• немного воды = a little water
• немного сахара = a little sugar
• немного времени = a little time

Since parsley here is an uncountable ingredient, the genitive is exactly what you expect.

Because петрушка here means parsley as a substance/herb, not separate countable items.

Russian treats many ingredients as mass nouns in this context, so singular is normal:

• немного сыра = a little cheese
• немного зелени = a little greenery/herbs
• немного петрушки = a little parsley

If you used a plural form, it would suggest separate countable units, which is not the idea here.

Yes. Russian word order is flexible, and different orders can shift emphasis.

The original sentence is neutral and natural:

• Мне кажется, что баклажан вкуснее, когда в соусе есть немного петрушки.

You could also hear variations like:

• Мне кажется, баклажан вкуснее, когда в соусе есть немного петрушки.
• Баклажан, мне кажется, вкуснее, когда в соусе есть немного петрушки.

These all mean roughly the same thing, but the emphasis changes slightly.

The given version is a good standard order for learners:

1. opinion frame: Мне кажется, что...
2. main idea: баклажан вкуснее
3. condition: когда в соусе есть немного петрушки

Not always. In conversation, Russian often drops что:

• Мне кажется, баклажан вкуснее...

This is very common and natural in speech.

But with что, the structure is clearer and slightly more explicit:

• Мне кажется, что баклажан вкуснее...

For learners, keeping что is often a good habit, because it makes the sentence easier to build correctly.