Pontus magnus est.

Latin usually does not use articles the way English does.

So Pontus magnus est can mean:

• Pontus is great
• The Pontus is great
• A sea is great (depending on context, though this is less likely here)

You normally figure out whether English should use the, a, or nothing at all from the context.

No. Latin word order is much more flexible than English word order.

In English, you usually need:

• subject + verb + complement
  → Pontus is great

In Latin, you can say:

• Pontus magnus est
• Pontus est magnus
• Magnus est Pontus
• Magnus Pontus est

All of these can mean basically the same thing, though the emphasis may change slightly.

Putting the verb at the end is very common in Latin, especially in simple textbook sentences.

Est is the third-person singular present tense of esse, which means to be.

So:

• sum = I am
• es = you are
• est = he/she/it is

Since Pontus is a singular subject, Latin uses est = is.

Because Latin adjectives must agree with the noun they describe.

They agree in:

• gender
• number
• case

Here, Pontus is:

• masculine
• singular
• nominative

So the adjective must also be:

• masculine
• singular
• nominative

That gives magnus.

Compare:

• magnus = masculine singular
• magna = feminine singular
• magnum = neuter singular

Here Pontus is in the nominative singular.

You can tell because it is the subject of the sentence — the thing being described as great.

In Pontus magnus est:

• Pontus = the subject
• magnus = describes the subject
• est = the linking verb, is

So Pontus and magnus are both nominative, because they refer to the same thing.

Because after a form of to be (esse), Latin often uses a predicate adjective in the nominative.

In English, we do the same idea:

• The sea is great

Here great describes the subject the sea.

In Latin, that descriptive adjective usually matches the subject in case as well, so:

• Pontus = nominative
• magnus = nominative

This is why both words have subject-type endings here.

It can be either, depending on context.

• pontus with a lowercase p can be a noun meaning sea
• Pontus with a capital P can be the proper name Pontus, such as the region by the Black Sea

Since your sentence starts with Pontus, the capital letter by itself does not prove much, because the first word of a sentence is capitalized anyway. You would need context to know whether it is meant as a name or as the common noun.

In a common reconstructed Classical pronunciation, it would be roughly:

• Pontus → PON-toos
• magnus → MAG-noos
• est → est

A rough full pronunciation: PON-toos MAG-noos est

Some details:

• u is pronounced like oo
• g is always a hard g, as in go
• gn is pronounced with both sounds, roughly like ngn or a tight gn

Yes, it is a complete sentence.

It has everything needed:

• Pontus = subject
• magnus = predicate adjective
• est = verb

So it fully means Pontus is great.

Latin can sometimes omit forms of to be, especially in certain styles, but here nothing is missing.

Yes.

Latin allows both:

• Pontus magnus est
• magnus Pontus est

Both are grammatical.

Often the choice depends on style or emphasis rather than strict grammar. Beginners will often first see adjective + noun or noun + adjective patterns, and both are normal in Latin.