Soror amicis parva munera dat, et una conviva valde gaudet.

Because amicis is in the dative plural.

With the verb dat = gives, Latin usually puts:

• the thing given in the accusative
• the person receiving it in the dative

So in Soror amicis parva munera dat:

• soror = the subject, the sister
• parva munera = the things being given, small gifts
• amicis = the recipients, to/for the friends

Because munera is accusative plural here, and it is the thing being given.

The verb dare means to give, so something must be given. In this sentence, that something is parva munera.

Also:

• munera is the plural of munus, muneris = gift
• neuter plural nominative and accusative often end in -a
• here the sense shows it is the accusative: the gifts are being given

So parva munera = small gifts is the direct object.

Because the adjective has to agree with the noun it describes in:

• gender
• number
• case

Here:

• munera is neuter plural accusative
• so the adjective must also be neuter plural accusative
• the neuter plural accusative form of parvus, -a, -um is parva

So:

• parva munera = correct
• parvos would be masculine plural accusative
• parvas would be feminine plural accusative

Neither of those matches munera.

Munera comes from munus, muneris, which is a third-declension neuter noun.

Its basic forms are:

• munus = singular nominative
• muneris = singular genitive

Important forms:

• singular: munus
• plural nominative/accusative: munera

This is very common for third-declension neuter nouns:

• corpus → corpora
• nomen → nomina
• munus → munera

So munera may look unusual at first, but it is a normal third-declension neuter plural form.

You know mainly from the case endings and the verb endings, not just from word order.

In the first clause:

• soror is nominative singular, so it is the subject
• dat is third person singular, so it matches soror

In the second clause:

• conviva is nominative singular, so it is the subject
• gaudet is third person singular, so it matches conviva

So the two subjects are:

• soror in the first clause
• una conviva in the second clause

Both are present active indicative, third person singular.

• dat comes from do, dare = to give
• gaudet comes from gaudeo, gaudere = to rejoice / be glad

So:

• dat = he/she/it gives
• gaudet = he/she/it rejoices / is glad

Because they are singular, each verb has a singular subject:

• soror gives
• conviva rejoices

Latin word order is much more flexible than English word order because Latin uses endings to show each word’s role.

English depends heavily on position:

• The sister gives small gifts to friends

Latin can rearrange the same idea in several ways, for example:

• Soror amicis parva munera dat
• Parva munera soror amicis dat
• Amicis soror parva munera dat

All of these can mean the same basic thing.

The chosen order may reflect style, emphasis, or rhythm, not a different grammar.

Here una means one or a single.

It is an adjective agreeing with conviva:

• una conviva = one guest or a single guest

This is the feminine form of unus, una, unum.

So una is not the adverb meaning together here. It is the numeral/adjective one.

It is a first-declension noun in form, but that does not automatically make it feminine.

Conviva, convivae means guest or dinner guest, and it can refer to a man or a woman. It is one of those Latin nouns that are first declension in form but not strictly feminine in meaning.

What tells you the gender here is una:

• una conviva = one female guest
• if it were male, you would expect unus conviva

So yes:

• conviva is first declension
• but it can still refer to a male person
• in this sentence, una shows that the guest is female

Valde is an adverb, and it modifies gaudet.

So:

• gaudet = is glad / rejoices
• valde gaudet = is very glad / rejoices greatly

It does not describe conviva. It describes the manner or intensity of the rejoicing.

In form, yes: amicis can be either dative plural or ablative plural.

But in this sentence, the verb dat strongly points to the dative meaning:

• to the friends

Latin learners often have to use context and the verb to decide between forms that look the same.

So although amicis could be ablative in another sentence, here it is best understood as dative plural.

Classical Latin does not have articles like English the and a/an.

So a Latin noun like soror can mean:

• sister
• a sister
• the sister

The exact English translation depends on the context.

That is why soror amicis parva munera dat can naturally be translated with articles in English even though Latin does not include separate words for them.