Thynnus magnus est, sed ostreum parvum est.

Because Latin adjectives must agree with the nouns they describe in gender, number, and case.

• thynnus is masculine singular nominative, so it takes magnus
• ostreum is neuter singular nominative, so it takes parvum

So the different endings do not change the basic meaning of the adjectives. They show which noun each adjective belongs to.

They are both in the nominative singular. In this sentence, each one is the subject of its own clause:

• Thynnus magnus est
• sed ostreum parvum est

The nominative case is commonly used for the subject of a sentence.

Latin word order is much more flexible than English word order. A very common pattern is to place the verb near the end of the clause, so magnus est and parvum est are perfectly natural.

English usually prefers is big and is small, but Latin often likes big is and small is in terms of word order. The meaning is still the same.

Sed means but. It joins the two clauses and shows contrast:

• the tuna is big
• but the oyster is small

It is a very common Latin conjunction.

Each half of the sentence is its own clause, so each clause has its own verb:

• Thynnus magnus est
• sed ostreum parvum est

Latin can sometimes leave out a form of to be when it is easily understood, especially in certain styles, but in a basic sentence like this, repeating est is normal and clear.

Latin does not have articles like English does. So thynnus can mean a tuna or the tuna, and ostreum can mean an oyster or the oyster. The context tells you which is meant.

That is why the Latin sentence can be translated in more than one natural English way.

The main clue is agreement. Latin adjectives match the nouns they describe.

• thynnus is masculine singular nominative, so magnus matches it
• ostreum is neuter singular nominative, so parvum matches it

Word order helps too, since each adjective is placed near its noun, but the endings are the most important clue.

Yes. Latin adjectives can come before or after the nouns they describe. So Latin could also have different word orders with the same basic meaning.

In this sentence, the adjectives come after the nouns:

• thynnus magnus
• ostreum parvum

That is a normal and common arrangement.

Because ostreum is a neuter noun. A neuter singular nominative adjective usually ends in -um, so the correct form is parvum.

If the noun were masculine, you would expect parvus instead. So the ending -um here is showing agreement with ostreum.

Yes. It is actually made of two complete clauses joined by sed:

• Thynnus magnus est
• sed ostreum parvum est

Each clause has a subject, an adjective describing the subject, and the verb est. So even though the sentence is simple, it is fully complete and grammatical.