Mia avino havas bonan recepton por supo kun tomatoj kaj cepoj.

Because recepton is the direct object of havas.

In Esperanto, the thing directly affected by the verb usually takes the accusative ending -n.

• havas = has
• What does she have? → recepton

So:

• Mia avino havas recepton. = My grandmother has a recipe.

That is why recepto becomes recepton here.

Because adjectives in Esperanto must agree with the nouns they describe.

Since recepton is:

• singular
• accusative (-n)

the adjective must match it:

• bona recepto = a good recipe
• bonan recepton = a good recipe (as a direct object)

So both words show the same grammar:

• bonan = adjective + accusative
• recepton = noun + accusative

This agreement is a very regular feature of Esperanto.

Because supo is not the direct object of the main verb.

The main verb is havas, and the direct object of havas is bonan recepton.

The phrase por supo means for soup. Here, supo is part of a prepositional phrase introduced by por, so it does not take -n.

Compare:

• Ŝi havas supon. = She has soup.
  Here supon is the direct object.

But in your sentence:

• Ŝi havas recepton por supo. = She has a recipe for soup.
  Here recepton is the direct object, not supo.

Por means for.

So:

• recepto por supo = recipe for soup

It shows purpose or intended use.

This is more natural than using de here. In Esperanto:

• por = for
• de = of, from, by

So recepto por supo means a recipe intended for making soup, while recepto de supo would sound much less natural for this meaning.

It most naturally describes supo:

• supo kun tomatoj kaj cepoj = soup with tomatoes and onions

So the whole phrase is:

• bonan recepton por supo kun tomatoj kaj cepoj
• a good recipe for soup with tomatoes and onions

In other words, the grandmother has a recipe for a kind of soup that includes tomatoes and onions.

Because kun tomatoj kaj cepoj comes right after supo, that is the most natural thing it connects to.

Because -j marks the plural in Esperanto.

• tomato = tomato

• tomatoj = tomatoes

• cepo = onion
• cepoj = onions

Esperanto uses the same plural ending for all nouns: -j.
That is much simpler than English, where plurals can change in different ways.

Esperanto has:

• no indefinite article like English a/an
• one definite article: la = the

So:

• recepto can mean a recipe or just recipe, depending on context
• la recepto means the recipe

In this sentence, no article is needed, so Esperanto simply says:

• bonan recepton = a good recipe

This is very normal.

Mia means my.

It is a possessive adjective, and like other adjectives in Esperanto, it can agree with the noun if needed.

Here:

• avino is singular
• it is not accusative

So we get:

• mia avino = my grandmother

If the noun were plural or accusative, the possessive could change too:

• miaj avinoj = my grandmothers
• miam avinon would be wrong; it should be mian avinon = my grandmother (as direct object)

So mia does change when grammar requires it; it just does not need to in this sentence.

Because of the word-building system in Esperanto.

• avin- = grandparent-related root
• avino = grandmother
• avo = grandfather

This is different from:

• maljuna virino = old woman

So avino specifically means grandmother, not simply an elderly woman.

Yes, Esperanto word order is fairly flexible, because endings show the grammar.

The normal order here is:

• Mia avino havas bonan recepton por supo kun tomatoj kaj cepoj.

But some rearrangements are possible, especially for emphasis.

For example:

• Bonan recepton por supo kun tomatoj kaj cepoj mia avino havas.

This is grammatical, but less neutral and more emphatic.

For learners, it is best to use the normal order first:

• subject + verb + object + other phrases

That will usually sound the most natural.

Esperanto pronunciation is very regular. A rough guide:

• Mia ≈ MEE-ah
• avino ≈ ah-VEE-no
• havas ≈ HAH-vas
• bonan ≈ BO-nan
• recepton ≈ re-TSEP-ton
• por ≈ por
• supo ≈ SOO-po
• kun ≈ koon
• tomatoj ≈ to-ma-TOY
• kaj ≈ kai (like English eye with a k in front)
• cepoj ≈ TSE-poy

A few useful sound notes:

• c is always ts
• j is like English y
• stress is always on the second-to-last syllable

So:

• a-VI-no
• re-CEP-ton
• to-ma-TOJ
• CE-poj

That regular stress pattern makes Esperanto much easier to pronounce consistently than English.