Avus senex moritur.

Because moritur is a deponent verb form.

The dictionary form is morior, mori, mortuus sum, meaning to die. Deponent verbs are verbs that are passive in form but active in meaning. So even though moritur looks like a passive ending, it means he dies or the old man/grandfather dies, not he is died.

In this sentence, moritur is:

• present tense
• 3rd person singular
• active in meaning

So moritur = he/she/it dies.

Both avus and senex are in the nominative singular.

They are both in this form because they refer to the subject of the sentence:

• avus = grandfather
• senex = old man / old

In this sentence, senex goes with avus, so both words must agree in case and number. Since the subject is singular, both are singular; since the subject is nominative, both are nominative.

Here it is functioning like an adjective describing avus.

The word senex can be:

• a noun: an old man
• or an adjective-like word: old

In Avus senex moritur, it means something like the grandfather, old, dies or more naturally the old grandfather dies.

So although senex often appears as a noun in dictionaries and texts, here it is describing avus.

In Latin, adjectives do not have to come before the noun. They can come:

• before the noun
• after the noun
• sometimes separated from the noun

So avus senex is perfectly normal Latin. Latin word order is more flexible than English because the endings show how words relate to each other.

English usually prefers old grandfather, but Latin can say avus senex without any problem.

Yes. Latin word order is flexible, so all of these are grammatically possible:

• Avus senex moritur
• Senex avus moritur
• Moritur avus senex

The basic meaning stays the same because the endings still show the subject.

However, changing the order can slightly change the emphasis:

• putting moritur first may emphasize the action: dies
• putting senex first may emphasize that he is old

So the original order is one natural way to say it, but not the only possible one.

Latin has no articles. That means it does not have separate words for the, a, or an.

So avus senex moritur could be translated, depending on context, as:

• the old grandfather dies
• an old grandfather dies
• old grandfather dies (less natural in English, but closer to the Latin structure)

English must choose an article, but Latin does not need one.

The ending tells you.

Moritur has the 3rd person singular ending, so it means:

• he dies
• she dies
• it dies

If it were they die, the form would be moriuntur.

Also, the subject words avus and senex are singular, so the verb must also be singular.

Because senex is not a regular first/second-declension adjective.

It belongs to a different pattern. Some Latin descriptive words do not follow the -us, -a, -um type. Senex is one of those words. Its forms are based on a different declensional pattern, and that is why its nominative singular masculine form is senex, not something like senus.

So a learner should not expect every adjective to look like bonus.

Grammatically, avus is the main noun, and senex adds description to it.

You can think of the structure like this:

• avus = the person being talked about
• senex = what kind of grandfather he is: old
• moritur = what he does: dies

So senex is not the core subject by itself here; it modifies avus.

A common classroom pronunciation would be:

AH-woos SEH-neks MO-ri-toor

More approximately in Classical Latin:

• avus = AH-wus
• senex = SEH-neks
• moritur = MO-ri-tur

A few helpful points:

• v in Classical Latin is pronounced like English w
• e is usually like e in met
• i is like ee in machine when long, but shorter here
• the stress in moritur falls on mo

Yes. Depending on context, senex can mean:

• old
• aged
• elderly
• an old man

So if the meaning already shown to the learner is something like The old grandfather dies, that is fine. In a different context, a translator might choose aged grandfather or elderly grandfather if that sounds better in English.