Mercator Carthagini et Massiliae saepe moratur, sed domi rarius est.

Because Latin often treats place names, and a few special words like domus (home), differently.

Here the words are in the locative, a case used for place where with certain nouns:

• Carthagini = at/in Carthage
• Massiliae = at/in Massilia
• domi = at home

So Latin does not say in Carthagine or in domo here.

A useful pattern to remember:

• place where: locative
  
  • Romae = at Rome
  • domi = at home
• motion toward: accusative without a preposition
  
  • Romam = to Rome
• motion away from: ablative without a preposition
  
  • Roma = from Rome

So this sentence is using a standard Latin way of saying in/at a city and at home.

They are all locative forms, but they come from different kinds of nouns.

• Carthagini: locative singular of Carthago, Carthaginis
  For many 3rd-declension city names, the locative singular ends in -i.
• Massiliae: locative singular of Massilia, -ae
  For 1st-declension singular city names, the locative usually looks like the genitive singular: -ae.
• domi: an irregular locative form of domus
  This is one of the common special locatives learners memorize early.

So although they all translate as in/at, their endings differ because the nouns belong to different declensions and patterns.

Yes, -ae can mean several things in Latin, which is why it can be confusing.

For a 1st-declension noun like Massilia, Massiliae can be:

• genitive singular: of Massilia
• dative singular: to/for Massilia
• nominative plural: Massilias / the Massilias
• locative singular: at/in Massilia

In this sentence, the meaning and construction show that Massiliae is locative singular, because it is paired with Carthagini and follows a verb of staying: moratur.

So here it definitely means at/in Massilia, not of Massilia.

Moratur comes from moror, morari, moratus sum, which is a deponent verb.

A deponent verb:

• has passive forms
• but an active meaning

So moratur looks like a passive form, but it means:

• he stays
• he lingers
• he spends time
• sometimes he delays

In this sentence, saepe moratur means he often stays or he often spends time in those places.

This is a very common thing in Latin, so it is worth getting used to early:

• sequitur = he follows
• loquitur = he speaks
• moratur = he stays

Because Latin is simply expressing the idea a little differently in the second clause:

• Carthagini et Massiliae saepe moratur = he often stays in Carthage and Massilia
• sed domi rarius est = but he is at home less often

Using est with domi is very natural: literally, he is at home less often.

Latin could have repeated moratur, but it does not need to. The shift to est is idiomatic and easy to understand. English often does the same sort of thing:

• He spends a lot of time in London, but he is at home less often.

So the second clause is not strange; it is just a different but perfectly normal way to say it.

Rarius is the comparative adverb of rare or infrequently.

So it means:

• more rarely
• more naturally in English here: less often

That fits the contrast with saepe (often):

• saepe = often
• rarius = less often

Latin comparatives do not always need an explicit than phrase. The comparison can be understood from context. Here the sense is:

• He often stays in Carthage and Massilia, but he is at home less often.

In other words, home is the place where he is found less frequently.

Because Latin word order is flexible, and putting Mercator first clearly sets up the topic: the merchant.

A very neutral analysis of the sentence is:

• Mercator = subject
• Carthagini et Massiliae = places
• saepe = adverb
• moratur = verb
• sed = but
• domi = at home
• rarius = less often
• est = is

Latin often places important information early or late for emphasis, but here the order is quite natural and readable:

• subject first
• then places and adverb
• then the verb

So beginning with Mercator is a straightforward way to say what kind of person we are talking about.

No. It means that those are two places where he often spends time, not that he is physically in both at once.

Latin often coordinates locations like this:

• Carthagini et Massiliae = in Carthage and Massilia

The natural sense is:

• he often stays in Carthage,
• and he often stays in Massilia,
• as opposed to being at home.

So the sentence is describing his general habits, not a single moment.

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

So mercator can mean:

• a merchant
• the merchant

Which one is best depends on context. If the meaning has already been given to the learner, then the translation probably chooses one based on the situation.

This is normal in Latin. You always decide from context whether the noun is definite or indefinite.

You should memorize it as a very common special form.

Domi is the locative of domus and means at home.

It belongs to a small group of place expressions that Latin learners usually learn as set phrases, such as:

• domi = at home
• militiae = in military service / on campaign
• ruri = in the countryside
• humi = on the ground

Because domi is extremely common, it is worth treating almost like a vocabulary item in its own right: domi = at home.