Tessellarius tesseram inserit.

Because tessellarius is a second-declension masculine noun in the nominative singular.

In this sentence, it is the subject, so Latin uses the nominative case. The ending -us is a very common nominative singular ending for second-declension masculine nouns.

So here, tessellarius means the mosaic worker / tile-layer as the one doing the action.

Because tesseram is in the accusative singular.

It is the direct object of the verb—the thing being inserted—so Latin puts it in the accusative case. For many first-declension nouns, the accusative singular ends in -am.

So:

• tessera = nominative singular
• tesseram = accusative singular

That change in ending helps show its job in the sentence.

In Latin, you often know this from the case endings, not from word order alone.

Here:

• tessellarius has the nominative ending, so it is the subject
• tesseram has the accusative ending, so it is the direct object

So even if the words were rearranged, the endings would still tell you who is doing the action and what is receiving it.

Inserit is a verb in the:

• present tense
• active voice
• indicative mood
• third person singular

So it means he/she/it inserts or is inserting, depending on context.

Because the subject is tessellarius, the full sense is the mosaic worker inserts ...

Latin verbs usually include the subject information in their endings.

The ending -it here tells you the verb is third person singular. That means:

• he inserts
• she inserts
• it inserts

Since the sentence already has the noun tessellarius as the subject, Latin does not need a separate pronoun like he.

Latin does not have articles like English the and a/an.

So a sentence like Tessellarius tesseram inserit can mean, depending on context:

• The mosaic worker inserts the tile
• A mosaic worker inserts a tile
• The mosaic worker is inserting a tile

English has to choose an article; Latin usually leaves that to context.

Less important than in English.

Latin word order is often more flexible because the endings show the grammatical roles. In this sentence, the order is:

• subject: tessellarius
• object: tesseram
• verb: inserit

That is a very normal Latin pattern. But Latin could rearrange the words for emphasis, and the meaning would still be understood because of the case endings.

Yes, they are related in meaning.

• tessera is the noun for a tile or small piece
• tessellarius is a person connected with such tiles, so a mosaic worker or tile-layer

So the sentence uses two related words: the worker and the tile.

You would usually look them up like this:

• tessellarius, -i = mosaic worker, tile-layer
• tessera, -ae = tile, tessera
• insero, inserere = insert, put in

The form in the sentence is not always the dictionary form:

• tessellarius already is a dictionary-style form
• tesseram would be looked up as tessera
• inserit would be looked up as insero

They belong to different declensions:

• tessellarius is second declension
• tessera is first declension

That is why their endings look different:

• second declension nominative singular masculine often ends in -us
• first declension accusative singular often ends in -am

Recognizing declensions helps you identify case and number more quickly.

Yes.

A Latin present tense verb often corresponds to either the English:

• simple present: inserts
• or progressive present: is inserting

Which one sounds better depends on context. Latin does not always make that distinction as clearly as English does.

The noun tesseram would change to the accusative plural form.

Since tessera is first declension:

• singular accusative: tesseram
• plural accusative: tesseras

So the sentence would become:

Tessellarius tesseras inserit.

That would mean the mosaic worker inserts tiles.

Then the subject would become plural, and the verb would also have to become plural.

For tessellarius:

• singular nominative: tessellarius
• plural nominative: tessellarii

And the verb would change from inserit to inserunt.

So:

Tessellarii tesseram inserunt.

That means the mosaic workers insert the tile.

A good step-by-step method is:

1. Find the verb: inserit
2. Identify its person and number: third person singular
3. Find the nominative noun: tessellarius → subject
4. Find the accusative noun: tesseram → direct object

So the structure is:

[Subject] tessellarius + [Object] tesseram + [Verb] inserit

That habit is very useful for reading Latin smoothly.