Aestate multi ad litus veniunt et in arena sedent.

Aestate is in the ablative case, and Latin often uses the ablative by itself to express time when something happens. So aestate means in summer or during the summer.

This is a very common Latin pattern:

• hieme = in winter
• nocte = at night
• tertia hora = at the third hour

So the sentence does not need a separate word for in before aestate.

Multi is nominative plural masculine. Here it means many people or many men, depending on context.

It agrees with the understood subject of the verbs veniunt and sedent:

• multi ... veniunt = many come
• multi ... sedent = many sit

Because Latin often uses a masculine plural form for a mixed group, multi can sometimes be translated more naturally as many people rather than strictly many men.

These use two different prepositions because they express two different ideas:

• ad litus = to the shore/beach
  ad shows motion toward something, and it takes the accusative case.
• in arena = in/on the sand
  in here shows location, not motion, so it takes the ablative case.

So:

• ad litus veniunt = they come to the beach
• in arena sedent = they sit in/on the sand

This is a very important Latin distinction:

• in + accusative = into, onto
• in + ablative = in, on

Because litus is a neuter third-declension noun. In many neuter nouns, the nominative and accusative singular have the same form.

So:

• nominative singular: litus
• accusative singular: litus

Even though the form does not change, the function does. After ad, it must be accusative, so ad litus means to the shore.

Arena is in the ablative singular: arenā, usually written without the long mark as arena.

It is ablative because it follows in in the sense of location:

• in arena = in the sand or on the sand

The noun arena, arenae is a first-declension noun.

Both are present tense, third person plural:

• veniunt = they come / they are coming
• sedent = they sit / they are sitting

Latin present tense can often be translated in more than one way in English depending on context.

The subject for both verbs is multi.

Latin word order is generally more flexible than English because the endings show the grammatical roles of words.

A literal order here is:

• In summer many to the shore come and in the sand sit.

But in English we would normally say:

• In summer many people come to the beach and sit in the sand.

Latin often places adverbial words like aestate early in the sentence, and verbs often come later. The meaning is carried mainly by the case endings and verb endings, not by a fixed word order.

Here et simply means and. It connects the two actions:

• veniunt = come
• sedent = sit

So the same group of people is doing both actions:

• Many come to the beach and sit in the sand.

There is nothing unusual about et in this sentence.

In idiomatic English, on the sand may sound more natural in many contexts, because people physically sit on top of the sand.

However, Latin in arena regularly covers what English might express as either in or on, depending on context. So both are reasonable, and the exact English wording depends on what sounds most natural in the translation.

Here are the likely dictionary forms:

• aestate → aestas, aestatis = summer
• multi → multus, multa, multum = much, many
• ad = to, toward
• litus → litus, litoris = shore, beach
• veniunt → venio, venire, veni, ventum = come
• et = and
• in = in, on, into, onto
• arena → arena, arenae = sand
• sedent → sedeo, sedere, sedi, sessum = sit

Looking words up this way is important because Latin words often appear in sentences in forms different from the dictionary form.

Yes, Latin could omit multi and simply say:

Aestate ad litus veniunt et in arena sedent.

The verb endings -unt and -ent already show a third person plural subject: they.

But adding multi makes the subject more specific:

• without multi = they
• with multi = many people / many

So multi is not required for grammar, but it adds meaning.