Mercator pondus panis et casei in libra spectat.

Because mercator is in the nominative singular, the case normally used for the subject.

• mercator = merchant
• it is a 3rd-declension noun
• its nominative singular is already mercator

So mercator ... spectat means the merchant ... looks at / examines.

Good question. Pondus is a neuter 3rd-declension noun, and in Latin neuter nouns often have the same form in the nominative and accusative singular.

So pondus could be either:

• nominative singular = weight
• accusative singular = weight

In this sentence, we understand it as the direct object because:

• mercator already works naturally as the subject
• spectat needs something being looked at or examined
• the phrase pondus panis et casei makes a clear object: the weight of the bread and cheese

Because they are in the genitive case, which often expresses of.

Here Latin uses:

• pondus panis = the weight of bread
• pondus casei = the weight of cheese

So pondus panis et casei means the weight of bread and cheese.

This is a very common Latin pattern:
noun + genitive = noun of something

Because some Latin nouns have forms that are identical in more than one case.

For panis:

• nominative singular: panis
• genitive singular: panis

So the form does not change here, even though the function does.

We know it is genitive because of the construction:

• pondus often takes a genitive
• et casei strongly suggests that panis is parallel to casei

So both panis and casei depend on pondus.

Because they belong to different declensions.

• panis, panis is 3rd declension
• caseus, casei is 2nd declension

Different declensions make the genitive singular differently:

• panis → genitive singular panis
• caseus → genitive singular casei

So they do the same grammatical job, but with different endings.

In libra is a prepositional phrase using in + ablative, which usually means in, on, or within without motion.

So here it means something like:

• on the scale
• in the balance
• on the weighing device

The noun libra can mean a balance / scale, and in some contexts also a pound. In this sentence, the sense connected with weighing is what matters.

Because Latin texts often leave out macrons.

The form here is ablative singular:

• nominative: libra
• ablative: librā

Without macrons, both are written libra.

So in a fully marked text, you would see in librā.

English often uses scales as a plural word, but Latin can use libra in the singular for a weighing instrument or balance.

Languages do not always match number the same way. A thing that is plural in English may be singular in Latin, and vice versa.

So singular libra is normal Latin usage here.

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

A very common Latin pattern is:

• subject ... object ... verb

So Mercator pondus panis et casei in libra spectat is perfectly normal.

Putting the verb last can make the sentence feel neat and orderly, but Latin could rearrange the words in other ways if needed.

It is joining panis and casei.

So the structure is:

• pondus
  • panis
  • et casei

In other words, it is the weight of bread and cheese, not two separate actions or clauses.

Because Classical Latin has no articles like English the and a/an.

Latin simply says:

• mercator
• pondus
• panis
• casei

Whether you translate these as a merchant, the merchant, bread, the bread, and so on depends on context.

Not literally. Spectat most basically means looks at, watches, or examines.

So in this sentence, the idea is that the merchant is looking at / inspecting the weight on the scale.

If Latin wanted to say more directly weighs, it might use a different verb. But spectat works well if the idea is that he is checking or observing the weight.