Mercator dicit mercem bonam esse.

Because mercator is the subject of dicit.

• mercator = the merchant
• dicit = says

So mercator is the person doing the saying, and subjects normally go in the nominative case.

Because mercem is in the accusative case, while merx is the nominative form.

In this sentence, Latin uses a very common construction after verbs like dicit: the accusative + infinitive construction for indirect statement.

So instead of saying:

• the merchant says that the merchandise is good

Latin literally structures it more like:

• the merchant says the merchandise to be good

That is why merx changes to mercem.

Esse is the infinitive form of to be.

After a verb like dicit, Latin often expresses reported speech or indirect statement with:

• an accusative subject
• plus an infinitive verb

Here:

• mercem = the subject of the indirect statement
• bonam = good
• esse = to be

So mercem bonam esse means that the merchandise is good, or more literally, the merchandise to be good.

Because bonam has to agree with mercem.

Mercem is:

• feminine
• singular
• accusative

So the adjective describing it must also be:

• feminine
• singular
• accusative

That gives bonam.

So:

• merx bona = the merchandise is good in a direct statement
• mercem bonam esse = the merchandise to be good / that the merchandise is good in an indirect statement

Not exactly.

It may look like a direct object at first, but grammatically it is better understood as the subject of the infinitive esse inside the indirect statement.

So the structure is:

• mercator = subject of dicit
• dicit = main verb
• mercem bonam esse = the whole reported statement

Within that reported statement:

• mercem is the subject
• bonam is its predicate adjective
• esse is the verb

So mercem belongs most closely with esse, not directly with dicit.

Because Latin usually does not use a conjunction like English that after verbs of saying, thinking, knowing, and so on.

English says:

• The merchant says that the merchandise is good.

Latin usually prefers:

• Mercator dicit mercem bonam esse.

So the idea of that is built into the accusative + infinitive construction rather than expressed by a separate word.

Because this is an indirect statement, not a direct statement.

If you said the idea directly, it would be:

• Merx bona est.
• The merchandise is good.

But after dicit, Latin changes the statement into indirect form:

• mercem bonam esse

So:

• est = is in a direct statement
• esse = to be in an indirect statement

The direct statement would be:

• Merx bona est.

That means The merchandise is good.

If you wanted to show the merchant saying those exact words, you could have something like:

• Mercator dicit: merx bona est.

But in ordinary reported speech, Latin usually changes it to:

• Mercator dicit mercem bonam esse.

Yes. Latin word order is more flexible than English word order because the endings show the grammatical roles.

So these can mean basically the same thing:

• Mercator dicit mercem bonam esse.
• Mercator mercem bonam esse dicit.
• Mercem bonam esse mercator dicit.

The exact order can change the emphasis a little, but the grammar stays clear because of the case endings and the infinitive.

It is an indirect statement, also called an accusative-and-infinitive construction.

This is one of the most important Latin sentence patterns. It is commonly used after verbs such as:

• dico = say
• puto = think
• scio = know
• audio = hear

The pattern is:

• accusative noun/pronoun
  • infinitive

Often there is also a predicate noun or adjective, as here:

• mercem = accusative subject
• bonam = predicate adjective
• esse = infinitive

It can mean either, depending on context.

Latin present tense often covers both:

• says
• is saying

So Mercator dicit mercem bonam esse could be translated as:

• The merchant says that the merchandise is good.
• The merchant is saying that the merchandise is good.

In most beginner contexts, says is the simplest translation.