Tessellarius tesseras aptat.

Because tessellarius is in the nominative singular, the case normally used for the subject of a sentence.

A learner can spot that because:

• tessellarius ends in -us
• that is a very common second-declension nominative singular ending for masculine nouns

Also, the verb aptat is third-person singular, so it matches a singular subject such as tessellarius.

Tesseras is accusative plural.

That tells you it is the direct object: it receives the action of the verb.

So in the sentence structure:

• tessellarius = subject
• tesseras = direct object
• aptat = verb

The ending -as is a very common sign of the accusative plural of a first-declension noun.

Aptat is:

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

It comes from the verb apto, aptare.

The ending -t is the key clue: it means he/she/it in the present tense.

So grammatically, aptat means something like:

• he fits
• he arranges
• he is fitting / arranging

The exact English wording depends on context.

Latin usually does not need a separate subject pronoun, because the verb ending already tells you the person and number.

Here, aptat already means he/she/it does.

So Latin can simply say:

• aptat = he/she/it arranges

The noun tessellarius is still included because it tells you who that he is.

Because Latin often likes to place the verb last, especially in simple prose.

So subject + object + verb is very common:

• Tessellarius = subject
• tesseras = object
• aptat = verb

English usually prefers subject + verb + object, but Latin word order is much more flexible because the endings show each word’s role.

Yes. Latin word order is flexible.

For example, these could still mean the same basic thing:

• Tessellarius tesseras aptat
• Tesseras tessellarius aptat
• Aptat tessellarius tesseras

The cases still show who is doing the action and what is being acted on.

However, changing the order often changes the emphasis:

• putting tesseras first may emphasize the tiles
• putting aptat first may emphasize the action

So word order in Latin is flexible, but it is not meaningless.

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

So a noun like tessellarius can mean, depending on context:

• the mosaic worker
• a mosaic worker

And tesseras can mean:

• the tiles
• tiles
• sometimes even some tiles

Latin leaves that to context.

Yes, they belong to the same word family.

• tessera = a small cube, tile, token
• tessellarius = a person connected with tessellae / tesserae, especially a tile-worker or mosaic worker

The suffix -arius often forms a noun meaning a person associated with something, often an occupation.

So tessellarius is essentially a tile-worker or mosaic worker.

This is also related to the English word tessellate.

A good way to store them is:

• tessellarius, -i = a mosaic worker / tile-worker
• tessera, -ae = a tile, small cube, token
• apto, aptare = to fit, fasten, arrange

If you learn the dictionary forms, it becomes much easier to recognize new endings later.

In a common reconstructed Classical pronunciation, approximately:

• tessellarius: tes-sel-LA-ri-us
• tesseras: TES-se-ras
• aptat: AP-tat

A few helpful points:

• the ss and ll are pronounced clearly as doubled consonants
• every vowel is pronounced
• Latin pronunciation is usually more regular than English spelling

So the whole sentence would sound roughly like:

tes-sel-LA-ri-us TES-se-ras AP-tat