Autumno multa folia in terra sunt.

Because Latin often uses the ablative of time when to say when something happens.
So autumno means in autumn or during autumn.

This is a very common pattern:

• aestate = in summer
• hieme = in winter
• vere = in spring
• autumno = in autumn

No preposition is needed here.

In English, we usually say in autumn, but Latin often leaves out in with expressions of time.

So:

• autumno = in autumn
• tertia hora = at the third hour
• eo die = on that day

This is normal Latin usage. A learner often expects a preposition because English uses one, but Latin does not always do that.

Because folia is a neuter plural noun, and the adjective has to agree with it in gender, number, and case.

• folium = leaf, singular, neuter
• folia = leaves, plural, neuter

The adjective multus, multa, multum changes form to match the noun:

• multi = masculine plural
• multae = feminine plural
• multa = neuter plural

So with folia, the correct form is multa.

The singular is folium, meaning leaf.

This is a second-declension neuter noun. Its basic pattern is:

• singular: folium
• plural: folia

That -a ending is very common for neuter plural nouns in Latin.

Here folia is nominative plural, because it is the subject of sunt.

You can tell this because:

• sunt means are, so it needs a plural subject
• folia is plural
• the sentence is saying that the leaves are somewhere

So folia is the thing being talked about: the leaves are on the ground.

Also, for many neuter nouns, the nominative plural and accusative plural look the same, so you often decide from the sentence structure.

Because in takes the ablative when it means in or on in the sense of location.

So:

• in terra = on the ground / in the ground / on the earth depending on context

Compare that with motion toward a place:

• in terram = into the ground / onto the ground

This is an important distinction:

• in + ablative = location
• in + accusative = motion into/toward

It can mean different things depending on context, because Latin terra has a range of meanings, including ground, land, and earth.

So in terra could be understood as:

• on the ground
• in the ground
• on the earth / on the land

In this sentence, with leaves in autumn, the most natural sense is usually on the ground.

Because the subject, folia, is plural.

• est = is
• sunt = are

So:

• folium in terra est = the leaf is on the ground
• folia in terra sunt = the leaves are on the ground

The verb must agree with its subject in number.

Latin word order is much more flexible than English word order. The endings on the words show their roles, so Latin does not depend as heavily on position.

Placing the verb at the end is very common in Latin, especially in simple prose sentences.

So this sentence could also be rearranged, for example:

• Multa folia in terra sunt autumno
• In terra multa folia sunt autumno

But Autumno multa folia in terra sunt is a perfectly natural Latin order.

No. Classical Latin does not have a definite article like English the or an indefinite article like a/an.

So folia can mean:

• leaves
• the leaves

And terra can mean:

• ground
• the ground

You decide from the context and the translation you have been given.

Here it means many, because folia is a plural count noun: you can count leaves.

So:

• multa folia = many leaves

By contrast, multum with a singular mass noun often means much:

• multum aquae = much water

So the exact English word depends on the kind of noun it modifies.

Yes. That is a very natural English translation.

Latin sunt can often be translated either as simple are or as there are, depending on what sounds normal in English.

So these are both reasonable ways to express the same idea:

• Many leaves are on the ground in autumn
• There are many leaves on the ground in autumn

The Latin structure itself does not need a separate word for there.