Marcus hodie abest, quia aegrotus est.

Because Marcus is the subject of the sentence, and subjects are put in the nominative case in Latin.

• Marcus = nominative singular
• Marcum would be accusative singular, which is usually used for a direct object, not the subject

So in Marcus hodie abest, Marcus is the one who is absent.

Hodie is an adverb, meaning today.

Adverbs modify verbs, adjectives, or other adverbs. Here it modifies abest and tells when Marcus is absent.

A useful thing to notice: adverbs like hodie do not change their form for case, number, or gender.

Abest is the 3rd person singular present indicative active form of absum, abesse, which means to be away, to be absent.

So:

• abest = he/she/it is absent or he/she/it is away

In this sentence, it refers to Marcus, so it means Marcus is absent.

Yes. Abest is a compound built from ab- plus est.

• est = he/she/it is
• abest = literally something like is away/from, which becomes is absent

This is part of the verb absum, an irregular compound of sum.

That is why abest may look familiar if you already know est.

Because in Latin this is treated as a compound verb, and compound verbs are normally written as one word.

So:

• correct: abest
• not standard: ab est

This is similar to other compounds of sum, such as:

• adest = is present
• deest = is lacking
• praeest = is in charge

Quia means because and introduces a subordinate clause of reason.

So the sentence has two parts:

• Marcus hodie abest = main clause
• quia aegrotus est = subordinate clause explaining why

In other words, quia connects the reason to the main statement.

Because Latin normally uses a form of sum (to be) with a predicate adjective, just as English does with is.

• aegrotus = sick
• aegrotus est = he is sick

So the full idea is expressed with both the adjective and the verb.

In some contexts Latin can omit forms of to be, especially in poetry or very compressed style, but in a normal beginner sentence like this, est is expected.

Because aegrotus is a predicate adjective describing Marcus, so it must agree with Marcus in:

• gender: masculine
• number: singular
• case: nominative

Since Marcus is masculine singular nominative, the adjective must also be masculine singular nominative:

• Marcus ... aegrotus est

If the subject were feminine, you would expect aegrota. If the subject were neuter, you would expect aegrotum.

Because it is not a direct object. It is a predicate nominative/predicate adjective linked to the subject by est.

With linking verbs like sum, Latin uses the nominative for the word that describes or identifies the subject.

So in:

• Marcus aegrotus est

both Marcus and aegrotus are nominative, because aegrotus describes Marcus.

Word order does not change that relationship.

Yes. The subject of est in quia aegrotus est is understood to be Marcus.

Latin often does not repeat a subject when it is already clear from context.

So the sentence works like this:

• Marcus hodie abest
• quia (Marcus) aegrotus est

English also sometimes does this in longer sentences, but Latin does it very naturally.

Yes. Latin word order is more flexible than English word order because the endings show the grammatical relationships.

So all of these could be understandable Latin:

• Marcus hodie abest, quia aegrotus est.
• Hodie Marcus abest, quia aegrotus est.
• Marcus abest hodie, quia aegrotus est.

However, the original order is a very natural, straightforward way to say it.

Putting a word earlier or later can change emphasis, but not necessarily the basic meaning.

Because the reason is presented as a real fact: Marcus is absent because he is sick.

So quia here introduces a clause with the indicative mood, which is the normal mood for stating facts.

In some Latin authors, especially in more advanced texts, clauses of reason may sometimes use the subjunctive if the reason is presented as someone’s claim, thought, or alleged explanation. But here the sentence is simple and factual, so est in the indicative is exactly what you would expect.

Primarily, aegrotus is an adjective meaning sick or ill.

In this sentence it is clearly an adjective describing Marcus:

• Marcus ... aegrotus est = Marcus is sick

But like many Latin adjectives, it can also be used substantively, meaning something like a sick man or a sick person, depending on context.

Here, though, it is best understood simply as an adjective.

In a classical pronunciation, you could say it approximately like this:

MAR-kus HO-di-eh A-best, KWI-ah ae-GRO-tus est

A few notes:

• c in Marcus is always hard, like k
• h in hodie is light
• qu in quia sounds like kw
• ae in aegrotus is usually pronounced like the ai in aisle in reconstructed classical pronunciation

Pronunciation systems vary somewhat, but those are the main beginner points.