Mi aĉetas freŝajn tomatojn kaj cepojn por la vespermanĝo.

The -n ending marks the direct object in Esperanto.

In this sentence, the thing being bought is fresh tomatoes and onions, so those words are the direct objects of aĉetas (buy/am buying).

• tomatoj = tomatoes
• tomatojn = tomatoes as a direct object
• cepoj = onions
• cepojn = onions as a direct object

The adjective also matches the noun, so:

• freŝaj tomatoj = fresh tomatoes
• freŝajn tomatojn = fresh tomatoes as a direct object

Since freŝajn describes the object, it also takes -n.

The ending -j marks the plural.

So:

• tomato = tomato
• tomatoj = tomatoes

and

• cepo = onion
• cepoj = onions

When a noun is both plural and a direct object, it gets both endings:

• -j for plural
• -n for direct object

So:

• tomatojn = tomatoes
• cepojn = onions

The adjective must match too:

• freŝajn = fresh, plural, direct object

Yes. freŝajn is understood to describe both nouns: tomatojn kaj cepojn.

Esperanto adjectives agree with the nouns they describe in:

• number: singular or plural
• case: whether they take -n

Because both nouns are plural direct objects, freŝajn is also plural and accusative.

So the phrase means:

• freŝajn tomatojn kaj cepojn = fresh tomatoes and onions

In context, it naturally means both the tomatoes and the onions are fresh.

Because that is the most common and natural order in Esperanto:

• freŝaj tomatoj = fresh tomatoes

Adjectives can sometimes come after the noun in Esperanto, but before the noun is more usual, especially in basic sentences.

So:

• Mi aĉetas freŝajn tomatojn sounds very natural.

Esperanto -as is the present tense. It does not force the same distinction English does between:

• buy
• am buying
• do buy

So Mi aĉetas can mean any of these depending on context:

• I buy
• I am buying
• I do buy

In this sentence, the most natural English translation is usually I am buying or I’m buying.

Esperanto often does not use an extra word where English might say some.

So:

• Mi aĉetas tomatojn kaj cepojn
  can naturally mean
• I’m buying tomatoes and onions
  or
• I’m buying some tomatoes and onions

The basic noun by itself is often enough.

Por means for.

Here it shows purpose:

• por la vespermanĝo = for dinner / for the evening meal

So the idea is that the speaker is buying the vegetables in order to use them for that meal.

La means the.

In Esperanto, la is used when the speaker has a specific thing in mind. Here, la vespermanĝo means a particular meal, essentially the dinner or tonight’s dinner, depending on context.

Without la, por vespermanĝo would mean something more like for dinner in a more general sense. Both can be possible in different contexts, but por la vespermanĝo points to a specific meal.

It is a compound word:

• vesper- = evening
• manĝo = meal, eating

So vespermanĝo literally means evening meal, which is usually translated as dinner or supper.

Compound words are very common in Esperanto.

Yes. Esperanto word order is fairly flexible because endings show grammatical roles.

This sentence uses a very normal order:

• Mi = subject
• aĉetas = verb
• freŝajn tomatojn kaj cepojn = object
• por la vespermanĝo = purpose phrase

Because tomatojn and cepojn have -n, you can still recognize them as the object even if the sentence is rearranged.

For example, these are possible:

• Por la vespermanĝo mi aĉetas freŝajn tomatojn kaj cepojn.
• Freŝajn tomatojn kaj cepojn mi aĉetas por la vespermanĝo.

But the original version is the most straightforward.

Because kaj simply means and, just like in English.

So:

• tomatojn kaj cepojn = tomatoes and onions

One kaj is enough to join the two nouns.

Yes. They are regular nouns.

Esperanto nouns normally end in -o:

• tomato = tomato
• cepo = onion

Then they change regularly:

• plural: -oj
• accusative: -on
• plural accusative: -ojn

So:

• tomato → tomatoj → tomatojn
• cepo → cepoj → cepojn

This regularity is one of the easiest parts of Esperanto grammar.

The letter ĉ is pronounced like ch in church.

So aĉetas sounds roughly like ah-CHE-tahs.

A quick breakdown:

• a = like a in father
• ĉ = ch
• e = like e in bet, but usually a bit clearer and purer
• t = plain t
• as = present-tense ending, pronounced clearly

Esperanto spelling is very regular, so once you know the letters, pronunciation is usually predictable.