Moja baka kaže da je blitva najbolja s uljem i češnjakom.

Because baka is a feminine singular noun, so the possessive adjective moj has to agree with it:

• moj = masculine singular
• moja = feminine singular
• moje = neuter singular

So:

• moj djed = my grandfather
• moja baka = my grandmother
• moje selo = my village

Croatian adjectives and possessives usually agree with the noun in gender, number, and case.

Kaže means says.

It is the 3rd person singular present tense form of the verb kazati or reći in the sense of to say.

So:

• ja kažem = I say
• ti kažeš = you say
• on/ona kaže = he/she says

In your sentence, baka is one person, so Croatian uses kaže:

• Moja baka kaže... = My grandmother says...

Da introduces a subordinate clause and often corresponds to English that.

So:

• Moja baka kaže da... = My grandmother says that...

In English, that is often optional:

• My grandmother says blitva is best...
• My grandmother says that blitva is best...

In Croatian, da is very commonly used in this structure.

Because je is the verb is, and the clause needs it:

• blitva je najbolja = blitva is the best

When this becomes a da-clause, Croatian often places the short form je immediately after da:

• da je blitva najbolja

This may feel unusual to an English speaker, because English keeps the subject before the verb:

• that blitva is the best

But in Croatian, short unstressed words like je often appear very early in the clause. This is part of normal Croatian clitic placement.

Because je is a clitic: a short unstressed form that tends to go in the second position of its clause.

In the clause:

• da je blitva najbolja

the word da occupies the first position, so the clitic je comes right after it.

This is very normal in Croatian. Compare:

• Mislim da je dobro. = I think that it is good.
• Kaže da je blitva najbolja. = She says that blitva is the best.

So the order is not random; it follows a common Croatian pattern.

Because najbolja agrees with blitva, which is feminine singular.

The adjective must match the noun:

• najbolji = masculine singular
• najbolja = feminine singular
• najbolje = neuter singular

Since blitva is feminine:

• blitva je najbolja

If the noun were masculine, you would get:

• kruh je najbolji = bread is the best

If it were neuter:

• vino je najbolje = wine is the best

Yes. It comes from dobar = good, but the comparison is irregular.

The forms are:

• dobar = good
• bolji = better
• najbolji = best

Then these forms change for gender:

• masculine: najbolji
• feminine: najbolja
• neuter: najbolje

So in your sentence:

• blitva je najbolja = blitva is the best

This is similar to English good → better → best, which is also irregular.

Blitva is in the nominative singular.

That is because it is the subject of the clause:

• blitva je najbolja = blitva is the best

The nominative is the basic dictionary form, and it is commonly used for the subject of a sentence.

Here s means with, and in this meaning it takes the instrumental case.

So:

• ulje → uljem
• češnjak → češnjakom

That gives:

• s uljem = with oil
• s češnjakom = with garlic

And together:

• s uljem i češnjakom = with oil and garlic

This is a very useful pattern to remember:

• s prijateljem = with a friend
• s obitelji = with family
• s mlijekom = with milk

Both s and sa can mean with, but sa is used mainly for easier pronunciation in certain sound combinations.

You often see sa before words beginning with sounds like s, š, z, ž, or in clusters that are hard to pronounce:

• sa sestrom = with my sister
• sa školom = with the school

But before uljem, simple s is natural:

• s uljem

So in your sentence, s uljem i češnjakom is the normal form.

Because both nouns are governed by the same preposition s and are linked by i = and.

So both have to be in the instrumental:

• s uljem
• i češnjakom

Croatian applies the same case to both coordinated nouns after the preposition.

Compare:

• s kruhom i sirom = with bread and cheese
• s majkom i ocem = with mother and father

Here blitva is being used like a food item in a general sense, similar to how English says spinach or chard.

So:

• Blitva je najbolja... = Chard is best...

It is singular because the sentence is talking about the dish/ingredient in general, not about individual pieces or plants.

It means that blitva is best when served/prepared with oil and garlic.

Croatian often leaves some things understood from context. A more expanded version might be:

• Blitva je najbolja kad je pripremljena s uljem i češnjakom.
• Blitva je najbolja uz ulje i češnjak.

But the shorter original sentence sounds natural and idiomatic.

Yes, Croatian word order is fairly flexible, although some versions sound more natural than others.

The original:

• Moja baka kaže da je blitva najbolja s uljem i češnjakom.

is completely natural.

You may also hear variations like:

• Moja baka kaže da je s uljem i češnjakom blitva najbolja.

That puts more emphasis on with oil and garlic.

However, the original version is the most straightforward and neutral for everyday use.

Yes: the sentence is very natural Croatian.

A few especially useful patterns are:

• X kaže da... = X says that...
• je najbolji/najbolja/najbolje = is the best
• s + instrumental = with + noun

So this one sentence contains several very common Croatian structures that are worth learning as patterns, not just as individual words.