Harundo longa est, et suber in aqua natat.

Because longa has to agree with harundo.

• harundo = reed
• longa = long

In Latin, adjectives agree with the noun they describe in gender, number, and case.
Harundo is feminine singular nominative, so the adjective must also be feminine singular nominative: longa.

So:

• harundo longa = a long reed / the reed is long

If the noun were masculine, you would expect longus. If it were neuter, longum.

Harundo is nominative singular.

It is the subject of est:

• Harundo longa est = The reed is long

The nominative case is normally used for the subject of a sentence and also for predicate nouns/adjectives connected with sum (to be).

It is part of is long.

In Harundo longa est, the structure is:

• harundo = subject
• longa = predicate adjective
• est = is

So literally: Reed long is.

Latin often uses an adjective with sum to say what something is like:

• puella laeta est = the girl is happy
• harundo longa est = the reed is long

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

So harundo can mean:

• a reed
• the reed

And suber can mean:

• a cork
• the cork

You decide from the context which English translation is best.

Aqua is ablative singular after in.

Here in means in / on in the sense of location, not movement. With that meaning, in takes the ablative:

• in aqua = in the water

This is different from in meaning into, which takes the accusative:

• in aquam = into the water

So:

• in aqua natat = it floats in the water
• in aquam cadit = it falls into the water

Natat is the 3rd person singular present active indicative of natare, meaning to swim or to float depending on context.

It is singular because the subject suber is singular:

• suber ... natat = the cork floats

If the subject were plural, you would expect natant:

• subera natant = the corks float

Because of the subject and context.

The verb natare basically means to swim, but with something like suber (cork), English naturally says float rather than swim. A cork does not actively swim; it stays afloat on the water.

So in this sentence, natat is best translated as floats.

Suber is neuter.

That is why, if it had an adjective with it, the adjective would need a neuter form.

For example:

• suber leve est = the cork is light

Even though suber does not end in -um, it is still neuter. In Latin, gender is something you often have to learn with the noun.

Et means and.

It simply joins the two clauses:

• Harundo longa est
• suber in aqua natat

So the full sentence is:

• The reed is long, and the cork floats in the water.

Yes. It is a perfectly normal word order, though Latin word order is more flexible than English.

This sentence is arranged as:

• Harundo longa est
• et suber in aqua natat

A few things to notice:

• The verb est often comes near the end.
• A prepositional phrase like in aqua can appear before the verb.
• Latin uses endings, so the order can vary more than in English.

For example, these would still be understandable Latin:

• Longa harundo est
• Suber natat in aqua

The exact order can affect emphasis, but the basic meaning stays the same.

A common reconstructed Classical pronunciation would be roughly:

• harundo longa est, et suber in aqua natat
• approximately: ha-ROON-do LONG-ga est, et SOO-ber in AH-kwa NA-tat

A few points:

• u sounds like oo
• g is always hard, as in go
• qu sounds like kw
• c would always be hard too, though there is no c here

If macrons are shown, the sentence would usually be written:

• Harundō longa est, et suber in aquā natat.

Those macrons show long vowels, which help with pronunciation and sometimes with grammar, but many Latin texts omit them.

Because the two clauses use different kinds of verbs.

In the first clause:

• Harundo longa est
• est is needed because longa is an adjective and Latin needs sum (to be) to say is long

In the second clause:

• suber in aqua natat
• natat already means floats, so no extra est is needed

So:

• longa est = is long
• natat = floats

They belong to different declensions:

• harundo, harundinis — 3rd declension, feminine
• suber, suberis — 3rd declension, neuter
• aqua, aquae — 1st declension, feminine

That matters because their endings change in different ways.

In this sentence:

• harundo = nominative singular
• suber = nominative singular
• aqua = ablative singular after in

So this sentence is also a good reminder that noun endings do not all work the same way in Latin; you need to learn each noun’s declension and gender.