Astrologus numerum computat.

Because astrologus is a second-declension masculine noun in the nominative singular. In this sentence, it is the subject, so Latin uses the nominative case.

So:

• astrologus = the astrologer / an astrologer as the subject

The -us ending is a very common nominative singular ending for masculine second-declension nouns.

Because numerum is the direct object of the verb, so it is in the accusative singular.

In this sentence, the astrologer is doing something to the number, so numerum is the thing being computed.

Compare:

• numerus = number as a subject
• numerum = number as a direct object

This is one of the most important patterns in Latin: the ending changes to show the word’s job in the sentence.

Computat is:

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

It means he computes, she computes, or it computes.

Because the subject here is astrologus, the natural translation is the astrologer computes.

The ending -at is a standard clue for third person singular present active in a first-conjugation verb.

We know from the case ending and from the verb ending.

• astrologus is nominative singular, so it is the subject
• computat means he/she/it computes
• numerum is accusative singular, so it is the object

Latin relies much more on endings than English does. English usually depends heavily on word order, but Latin often uses word endings to show who does what.

Latin usually does not use articles like English the or a/an.

So astrologus can mean:

• the astrologer
• an astrologer

And numerum can mean:

• the number
• a number

You figure out which is best from the context.

Not as much as in English.

Astrologus numerum computat is a perfectly normal Latin sentence, but Latin could also say:

• Numerum astrologus computat
• Computat astrologus numerum
• Numerum computat astrologus

These all basically mean the same thing, because the endings show the grammatical roles.

That said, word order in Latin can still affect emphasis. For example, putting numerum first might emphasize the number.

Because the verb ending already includes that information.

Computat already means he/she/it computes, so Latin does not need to add a pronoun like he unless it wants emphasis or contrast.

English says:

• He computes

Latin can simply say:

• Computat

The subject is often still expressed with a noun, as in astrologus numerum computat, but it does not have to be.

It is a Greek-derived word used in Latin. Many educated or technical words in Latin come from Greek.

So astrologus is not surprising in form, even though it follows normal Latin declension patterns here. A learner may notice that it looks a lot like English astrologer and related words in other European languages.

You would usually look them up like this:

• astrologus, astrologi = astrologer
• numerus, numeri = number
• computo, computare, computavi, computatum = compute, reckon, calculate

For nouns, dictionaries normally give the nominative singular and genitive singular.
For verbs, they normally give the principal parts.

A common classroom pronunciation would be roughly:

as-TROL-o-gus NOO-me-rum KOM-poo-taht

More traditionally in reconstructed Classical Latin:

• Astrologus = as-TRO-lo-gus
• numerum = NOO-me-rum
• computat = KOM-poo-taht

A few helpful points:

• c is always hard, like k
• u is pronounced like oo
• g is always hard, as in go

Yes. Latin verbs often cover a range of meanings that overlap with several English words.

Depending on context, computat could be translated as:

• computes
• calculates
• counts
• reckons

So even if one English meaning has been given already, it is useful to know that the Latin verb may be a little broader than a single English word.

It is:

• subject: astrologus
• object: numerum
• verb: computat

So the pattern is Subject + Object + Verb, which is a very common Latin order.

English more often prefers Subject + Verb + Object, but Latin frequently places the verb at the end.