Sermo oratoris civibus gratus est.

The core structure is:

• Sermo = the subject
• gratus est = the predicate, is pleasing / welcome
• civibus = the person(s) to whom it is pleasing
• oratoris = a genitive noun modifying sermo

So the sentence is built like this:

[The speech] [of the orator] [is pleasing] [to the citizens].

Because oratoris is in the genitive singular. The genitive often shows possession or close relationship, and one of its most common English equivalents is of ... or sometimes ...’s.

So:

• orator = orator / speaker
• oratoris = of the orator or the orator’s

That means sermo oratoris is the speech of the orator or the orator’s speech.

Civibus is dative plural from civis, meaning citizen.

The dative case often marks:

• the indirect object
• the person to whom something is given, said, or shown
• the person for whom something is pleasant, useful, harmful, etc.

Here it goes with gratus est, because in Latin gratus commonly takes a dative of the person pleased.

So:

• cives would be the citizens as a subject or direct object in some contexts
• civibus means to/for the citizens

Latin often expresses this idea with an adjective plus esse:

• gratus = pleasing, welcome, agreeable
• est = is

So civibus gratus est literally means is pleasing to the citizens or is welcome to the citizens.

This is a normal Latin way of expressing what English might also say as pleases the citizens.

Because gratus agrees with sermo.

In Latin, adjectives agree with the nouns they describe in:

• gender
• number
• case

Here:

• sermo is masculine singular nominative
• so the adjective must also be masculine singular nominative
• therefore we get gratus

If the subject were neuter, you would expect gratum; if feminine, grata.

Sermo is the nominative singular form of a masculine noun meaning things like:

• speech
• conversation
• discourse
• talk

In this sentence it is the subject, because it is the thing being described as gratus.

So sermo is the thing that is pleasing.

The cases tell you.

• sermo is nominative, so it is the subject: the thing that is pleasing
• civibus is dative, so it marks the people to whom it is pleasing

So the meaning is:

• the speech is pleasing to the citizens

not:

• the citizens are pleasing to the speech

Latin relies heavily on case endings for this.

Formally, yes: civibus can be either dative plural or ablative plural.

But in this sentence, the meaning and construction show that it is dative. The adjective gratus regularly takes a dative of the person to whom something is pleasing.

So here civibus means to the citizens, not something ablative like by/with/from the citizens.

In this sentence, the most natural interpretation is that oratoris depends on sermo and identifies whose speech it is:

• sermo oratoris = the orator’s speech

A genitive can sometimes express different relationships, but possession or authorship is the obvious one here.

Latin word order is much more flexible than English word order because the endings show the grammatical relationships.

English usually depends strongly on order:

• The orator’s speech is pleasing to the citizens

Latin can arrange the same parts more freely:

• Sermo oratoris civibus gratus est
• Civibus sermo oratoris gratus est
• Gratus est civibus sermo oratoris

These all keep basically the same meaning, though the emphasis may shift.

In your sentence, the order is quite natural:

• subject first
• then its genitive modifier
• then the dative
• then the adjective and verb

No. That is a very common point of confusion.

Gratus can sometimes be related to the English word grateful, but here it does not mean that the speech is feeling gratitude.

Here it means:

• pleasing
• welcome
• acceptable
• agreeable

So civibus gratus est means is pleasing to the citizens, not is grateful to the citizens.

Latin often does include est, especially in straightforward prose like this.

• est = is
• it is the 3rd person singular present of esse

Sometimes Latin can omit forms of esse, especially in certain styles or contexts, but with a beginner sentence like this, keeping est makes the structure very clear.

No. Latin has no definite article (the) and no indefinite article (a/an).

So:

• sermo could mean speech, a speech, or the speech
• oratoris could mean of an orator or of the orator
• civibus could mean to citizens or to the citizens

In practice, context determines what sounds best in English. In this sentence, the orator’s speech is pleasing to the citizens is the most natural translation.