Mater addit sermonem urbanum non semper meliorem esse quam sermonem rusticum.

Because after addit Latin uses an indirect statement construction.

In English we say:

• Mother adds that urban speech is not always better than rustic speech.

In Latin, instead of using that, Classical Latin usually makes the subject of the indirect statement accusative and the verb infinitive:

• sermonem urbanum ... esse

So sermonem urbanum is the subject of esse, but in an indirect statement that subject appears in the accusative.

Esse is the present infinitive of sum, meaning to be.

It appears because addit introduces an indirect statement. Latin commonly expresses this pattern as:

• verb of saying/thinking + accusative subject + infinitive

So:

• Mater addit = Mother adds / says further
• sermonem urbanum ... esse = that urban speech is ...

Without esse, the indirect statement would be incomplete.

Because meliorem agrees with sermonem urbanum, which is accusative singular masculine.

Since sermonem urbanum is the subject of the infinitive esse in indirect statement, any predicate adjective describing it must match it in:

• case
• number
• gender

So:

• sermo urbanus → nominative
• sermonem urbanum → accusative
• therefore melior becomes meliorem

It means better and agrees with sermonem urbanum.

Because with comparisons, the thing compared after quam is usually put in the same case as the thing it is being compared with.

Here the comparison is between:

• sermonem urbanum
  and
• sermonem rusticum

Since sermonem urbanum is accusative, sermonem rusticum is also accusative.

So:

• meliorem esse quam sermonem rusticum
  = to be better than rustic speech

It means not always.

So the sentence does not say that urban speech is never better. It says only that it is not better in every case.

That is an important difference:

• non semper meliorem esse = not always to be better
• not semper non meliorem esse in the sense always not better

Latin word order can be flexible, but here non semper naturally goes together as not always.

Yes. Addit is the present tense of addere, meaning to add.

In a sentence like this, it often means:

• adds
• adds the point that
• goes on to say

So Mater addit can be understood as:

• Mother adds
• Mother further says
• Mother adds that...

Because Classical Latin often does not use a separate word equivalent to English that after verbs of saying, thinking, knowing, and so on.

Instead, it uses the accusative-and-infinitive construction:

• English: Mother adds that urban speech is not always better...
• Latin: Mater addit sermonem urbanum non semper meliorem esse...

So the idea of that is built into the grammar of:

• accusative subject
  • infinitive

Not in the ordinary sense.

The direct object of addit is really the whole indirect statement:

• sermonem urbanum non semper meliorem esse quam sermonem rusticum

Inside that indirect statement, sermonem urbanum is the subject of esse.

So although sermonem urbanum is accusative, it is not simply the object Mother adds. It is the subject of the infinitive within reported speech.

Latin word order is much freer than English word order because Latin endings show grammatical relationships.

A more English-like order would be something like:

• Mater addit sermonem urbanum esse non semper meliorem quam sermonem rusticum.

But Latin often places:

• the infinitive esse toward the end,
• the negative phrase non semper before the adjective it affects,
• and the compared noun after quam.

So the given order is perfectly natural Latin.

Sometimes Latin can omit a repeated noun if the meaning is clear, but here repeating sermonem makes the comparison very clear and balanced:

• sermonem urbanum ... quam sermonem rusticum

That helps the reader see immediately that the comparison is between two kinds of sermo.

So the repetition is not strange; it is a clear and normal way to write the sentence.

Meliorem is:

• accusative
• singular
• masculine

It matches sermonem urbanum, because sermo is masculine and singular, and in this indirect statement it is accusative.

So the agreement is:

• sermonem — accusative singular masculine
• urbanum — accusative singular masculine
• meliorem — accusative singular masculine

All three go together grammatically.

Latin has two common ways to express comparison:

• comparative adjective + quam
• comparative adjective + ablative

For example, Latin can say either:

• meliorem quam sermonem rusticum
• or, in some contexts, a comparative with the ablative

Here quam is used, which is very straightforward and especially clear because the compared noun has modifiers and is explicitly repeated.

So quam sermonem rusticum is a normal and clear way to say than rustic speech.

A helpful way to see it is this:

• Mater addit
  = main clause

Then the indirect statement:

• sermonem urbanum = accusative subject
• meliorem esse = infinitive phrase
• non semper = modifies meliorem esse
• quam sermonem rusticum = comparison

So the structure is:

• Mother adds
• [urban speech] [not always to be better] [than rustic speech]

That is the core pattern of the sentence.