Templum dei magnum est.

Because magnum has to agree with templum.

• templum is nominative singular neuter
• so the adjective must also be nominative singular neuter

That is why Latin uses magnum, not the masculine form magnus.

We know because templum is in the nominative case, which is the normal case for the subject. It also matches magnum, which is a predicate adjective agreeing with it.

So the structure is:

• templum = subject
• dei = genitive dependent on templum
• magnum est = is great / is large

Because Latin word order is much freer than English word order. It is very common for Latin to place the verb near the end, especially in simple statements.

So Templum dei magnum est is perfectly normal Latin. If the words were rearranged, the basic meaning could stay the same, though the emphasis might change.

Latin has no word for the or a/an, so templum can mean the temple, a temple, or just temple, depending on context.

As for of, Latin does not use a separate word here. The idea of of is built into the genitive form dei.

It can mean either.

The adjective magnus, magna, magnum can refer to:

• size: big, large
• importance or quality: great

So the exact shade of meaning depends on context.

Here it is part of the predicate with est. In other words, the sentence means that the temple is great/large.

So this is a predicate adjective construction:

• templum = the thing being talked about
• magnum = what is being said about it
• est = links them

That is different from using magnum simply inside a noun phrase, as in the great temple.

Yes. Latin itself does not mark that difference clearly here.

Because Latin has no articles, dei can mean:

• of a god
• of the god
• of God

The context tells you which meaning is intended. In older Latin writing, capitalization does not help, because ancient texts did not use capitals the way modern English does.

They are:

• templum ← templum, templi = temple
• dei ← deus, dei = god
• magnum ← magnus, magna, magnum = great, large
• est ← sum, esse = am / is / are; to be

So this sentence contains a noun, a genitive noun depending on it, an adjective agreeing with the subject, and the verb to be.