Mater dicit vestimenta umida sub sole citius siccari.

Because Latin usually does not use a separate that-clause after verbs of saying, thinking, knowing, and perceiving.

Instead, it uses an accusative + infinitive construction:

• Mater dicit = Mother says
• vestimenta umida sub sole citius siccari = the wet clothes dry / are dried faster in the sun

So where English says Mother says that ..., Latin normally says Mother says [something] to be ....

This construction is often called indirect statement.

In Latin indirect statement, the subject of the infinitive goes into the accusative case.

So in this sentence:

• Mater is the subject of dicit
• vestimenta is the subject of siccari

But because siccari is part of an indirect statement after dicit, its subject appears in the accusative:

• vestimenta = accusative plural neuter

So vestimenta is the logical subject of siccari, even though it is not nominative.

Siccari is the present passive infinitive of siccare, meaning to dry or more literally to be dried.

It is used because after dicit, Latin normally puts the reported statement into an infinitive clause.

So:

• dicit ... siccari = says ... to dry / to be dried

The present infinitive usually shows action happening at the same time as the main verb, or a general truth. Here it gives the idea:

• Mother says that wet clothes dry faster in the sun

Good question. Siccare is usually a transitive verb, meaning to dry something. Its passive infinitive siccari literally means to be dried.

In English, we often say clothes dry. Latin can express the idea with the passive form here, especially in a general statement like this.

So literally the Latin is close to:

• Mother says that the wet clothes are dried more quickly in the sun

But natural English usually prefers:

• Mother says that wet clothes dry faster in the sun

Because umida is an adjective modifying vestimenta, and Latin adjectives must agree with the nouns they describe in:

• gender
• number
• case

Here:

• vestimenta = neuter plural accusative
• umida = neuter plural accusative

So vestimenta umida means wet clothes.

Even though vestimenta is the subject of the infinitive in meaning, it is still accusative in form because of indirect statement, and umida matches that.

Because sub can take either the ablative or the accusative, depending on the meaning.

• sub + ablative = location, under / in
• sub + accusative = motion toward a position under something

Here there is no movement; the idea is location:

• sub sole = under the sun / in the sun

So sole is ablative singular.

Literally, yes: sub sole means under the sun.

But in natural English, the best translation is usually:

• in the sun
• in the sunlight

Latin often uses sub in places where English idiom prefers in.

So you should understand sub sole here as in the sun, not as something unusually poetic or strange.

Citius is the comparative adverb, meaning:

• more quickly
• faster

By contrast, cito means simply:

• quickly

So the sentence is saying that the clothes dry faster in the sun than they do in some other condition.

Latin often uses a comparative without stating both sides explicitly. The comparison is understood:

• faster than otherwise
• faster than elsewhere
• faster than when not in the sun

Because classical Latin has no articles.

So mater can mean:

• mother
• a mother
• the mother

And vestimenta umida can mean:

• wet clothes
• the wet clothes
• sometimes even some wet clothes

The context tells you which English article makes the most sense.

Latin word order is much more flexible than English word order, because the endings show how the words relate to each other.

This sentence is arranged in a very natural Latin way:

• Mater dicit first gives the main statement
• the infinitive siccari comes at the end, which is common
• vestimenta umida stays together as a phrase

A different order could still be grammatical, for example:

• Mater vestimenta umida sub sole citius siccari dicit

That would still mean the same thing.

So the order in the original is not the only possible one, but it is a normal and readable Latin order.

Yes. Vestimentum means a garment or an item of clothing. Its plural vestimenta means clothes / garments.

That works well with English, because clothes is also plural in form.

So:

• vestimentum = garment
• vestimenta = clothes

In this sentence, Latin is talking about clothes in general, so the plural is natural.