Decempeda longa est, sed libella parva est.

Because they have to agree with the nouns they describe.

• decempeda is a feminine singular noun, so its adjective is longa
• libella is also a feminine singular noun, so its adjective is parva

In Latin, adjectives change form to match the noun in:

• gender
• number
• case

So this is the same basic idea as saying:

• feminine singular noun → feminine singular adjective

That is why you see longa, not longus or longum, and parva, not parvus or parvum.

They are all nominative singular.

Here is why:

• decempeda is the subject of est
• longa describes the subject, so it is also nominative
• libella is the subject of the second est
• parva describes that subject, so it is nominative too

In sentences with est meaning is, Latin often uses:

• subject in the nominative
• predicate adjective in the nominative

So:

• decempeda longa est = the decempeda is long
• libella parva est = the libella is small

Because Latin does not have articles like English does.

English usually needs words such as:

• the
• a
• an

Latin normally leaves that idea unstated, and you understand it from context.

So decempeda longa est could mean:

• the measuring rod is long
• a measuring rod is long

depending on the situation.

That can feel strange to English speakers at first, because English uses articles constantly, but Latin simply does not.

Because Latin word order is much more flexible than English word order.

In English, we normally say:

• a long rod
• a small level

In Latin, adjectives can come:

• after the noun
• before the noun
• separated from the noun

So decempeda longa and longa decempeda are both possible.

In this sentence, noun + adjective is perfectly normal:

• decempeda longa
• libella parva

The meaning does not change just because the adjective comes after the noun.

Because the sentence has two separate clauses:

• Decempeda longa est
• sed libella parva est

Each clause has its own subject and its own est.

Latin can sometimes leave out a repeated est if the meaning is obvious, but repeating it is very normal and clear, especially for learners and in simple prose.

So this sentence is structured very neatly:

• first statement: the decempeda is long
• contrast: but the libella is small

Sed means but.

It joins the two clauses and shows a contrast:

• Decempeda longa est = one fact
• sed libella parva est = a contrasting fact

So sed is a coordinating conjunction, just like English but.

It tells the reader:

• do not just connect the ideas
• compare them in contrast

Yes, very often.

Because Latin marks grammatical roles with endings, not mainly with position, you can rearrange the words. For example, these would still mean basically the same thing:

• Longa decempeda est, sed parva libella est.
• Decempeda est longa, sed libella est parva.
• Parva est libella, sed decempeda longa est.

What changes is usually emphasis, not core meaning.

For example:

• Longa decempeda est puts more attention on longa
• Parva est libella puts more attention on parva

So Latin word order is flexible, but not random: writers use it to highlight different parts of the sentence.

They are first-and-second-declension adjectives.

Their dictionary forms are:

• longus, longa, longum = long
• parvus, parva, parvum = small

In this sentence, the feminine nominative singular forms are needed, so we get:

• longa
• parva

That matches the feminine nouns:

• decempeda
• libella

This is a very common adjective pattern in Latin, so it is worth learning well.

Yes, in this sentence they appear as first-declension nominative singular forms.

They both end in -a, which is the usual nominative singular ending for many first-declension feminine nouns.

So a learner will often recognize:

• decempeda → feminine singular noun
• libella → feminine singular noun

That is one reason the matching adjectives also end in -a:

• longa
• parva

Because Latin commonly uses esse (to be) with an adjective, just as English does:

• est longa = is long
• est parva = is small

This is called a linking verb use of to be. The verb does not show an action here; it connects the subject to a description.

So:

• decempeda longa est literally works as the rod long is
• but in natural English we say the rod is long

That is a normal Latin sentence pattern.

Sometimes, yes.

Latin often omits forms of esse when the meaning is clear, especially in certain styles or shorter expressions. So you may sometimes see something like:

• Decempeda longa, sed libella parva

However, with beginners' Latin and with straightforward prose, keeping est is clearer and more common for teaching.

So in this sentence, the full form with est twice is completely normal and helpful.