Perpendiculum rectum pendet, sed canalis declivis est.

Because perpendiculum is a neuter singular noun, and the adjective has to agree with it.

• perpendiculum = neuter singular
• rectum = neuter singular form of rectus, recta, rectum

So rectum matches perpendiculum in gender, number, and case.

If the noun were masculine, you would expect rectus; if feminine, recta.

Here perpendiculum is nominative singular, because it is the subject of pendet.

The basic structure is:

• perpendiculum = the subject
• pendet = hangs
• rectum = describes the subject

So the first clause means something like the plumb line hangs straight/upright.

It is best understood as an adjective, not an adverb.

It agrees with perpendiculum, so it is describing the subject: the plumb line is straight/upright while hanging.

A native English speaker may expect an adverb because English often says things like hangs straight, where straight can feel adverb-like. But in Latin, rectum is still an adjective agreeing with the noun.

So the sense is not hangs straightly, but rather hangs in a straight/upright condition.

Because the first clause already has a full verb: pendet.

• pendet = hangs
• rectum = predicate adjective describing the subject while it hangs

Latin can use an adjective with certain verbs, not only with sum (to be). So rectum pendet means hangs straight/upright.

In the second clause, declivis est uses est because the idea is simply is sloping/inclined.

So the two clauses are built differently:

• perpendiculum rectum pendet = the plumb line hangs straight
• canalis declivis est = the channel is sloping

Pendet is:

• 3rd person singular
• present tense
• active voice
• from pendeo, pendere = to hang

It is singular because its subject, perpendiculum, is singular.

So pendet means it hangs or hangs.

Because declivis agrees with canalis.

Here canalis is nominative singular, and declivis is a third-declension adjective whose nominative singular masculine/feminine form is declivis.

So:

• canalis = channel
• declivis = sloping, downward-inclined
• est = is

Together: the channel is sloping.

A learner may expect something like declivus, but declivis belongs to a different adjective pattern.

It is nominative singular here, because it is the subject of est.

Even though canalis ends in -is, it is not automatically genitive or dative. It is a third-declension noun, and many third-declension nominative singular forms end in -is.

So in this sentence:

• canalis = subject
• declivis = adjective agreeing with it
• est = verb

Sed means but.

It joins the two clauses and shows a contrast:

• Perpendiculum rectum pendet
• sed
• canalis declivis est

So the idea is that the plumb line is vertical/straight, but the channel itself is sloping.

Latin word order is more flexible than English word order because Latin uses endings to show grammatical relationships.

In this sentence, the order helps highlight the two subjects and their descriptions:

• Perpendiculum rectum pendet
• sed canalis declivis est

English usually depends more heavily on word order, but Latin can move words around more freely without causing confusion.

Even so, this sentence is quite natural and clear:

• subject first
• descriptive word near the subject
• verb at or near the end

That is a very common Latin pattern.

Yes.

• perpendiculum is related to English perpendicular and can refer to a plumb line
• canalis is clearly related to English canal and channel
• rectum comes from rectus, related to English words like direct, correct, and rectangle
• declivis is related to the idea of a declivity, a downward slope

These connections can help with vocabulary, but it is still important to learn the exact Latin meaning in context.

The first clause is:

• perpendiculum = subject
• rectum = predicate adjective describing the subject
• pendet = main verb

So: subject + predicate adjective + verb

The second clause is:

• canalis = subject
• declivis = predicate adjective
• est = linking verb

So: subject + predicate adjective + linking verb

That makes the sentence a nice example of two slightly different ways Latin can describe a subject:

• with a normal action verb: rectum pendet
• with sum: declivis est