Avus meminit se olim lupum ferocem in monte vidisse.

Because memini, meminisse is a special verb. It is a defective verb whose perfect forms have a present meaning.

So:

• meminit = he/she remembers
• meminerat = he/she remembered
• meminerit = he/she will remember or sometimes may remember, depending on context

So in this sentence, Avus meminit means Grandfather remembers, not Grandfather remembered.

Se is the reflexive pronoun, here in the accusative singular. It refers back to the subject avus.

So se ... vidisse means that he had seen or himself to have seen, where he = the grandfather.

Latin uses se when the person inside the subordinate idea is the same as the subject of the main verb.

So:

• Avus meminit se vidisse = Grandfather remembers that he himself saw
• If it were someone else, Latin would use something like eum instead.

Because Latin often uses an indirect statement after verbs of thinking, saying, knowing, remembering, and similar ideas.

Here the structure is:

• Avus meminit = main clause
• se ... vidisse = indirect statement

In Latin, an indirect statement is usually built with:

• an accusative subject
• an infinitive

So instead of saying something like Grandfather remembers that he saw... with a separate finite verb, Latin says Grandfather remembers himself to have seen...

That is why vidisse appears.

The perfect infinitive shows that the seeing happened before the remembering.

So:

• meminit ... vidisse = remembers having seen / remembers that he saw
• If Latin used a present infinitive like videre, it would suggest something more like remembers that he is seeing, which would not fit here.

So vidisse is exactly what Latin needs to show an earlier completed action.

They are accusative for different reasons.

• se is accusative because it is the subject of the infinitive in an indirect statement
• lupum is accusative because it is the direct object of vidisse

So in se lupum ferocem ... vidisse:

• se = the one who did the seeing
• lupum ferocem = what he saw

This is a very common Latin pattern and can feel strange to English speakers, because English does not normally mark the subject of a subordinate clause this way.

Ferocem is an adjective modifying lupum.

It agrees with lupum in:

• gender: masculine
• number: singular
• case: accusative

So:

• lupus ferox = a fierce wolf as subject
• lupum ferocem = a fierce wolf as object

The ending changes because the noun is accusative singular masculine, and the adjective must match it.

Because in takes different cases depending on meaning:

• in + ablative = in/on a place, showing location
• in + accusative = into/onto a place, showing motion toward

Here the wolf was seen on the mountain, not into the mountain, so Latin uses the ablative:

• in monte = on the mountain

If someone were climbing onto the mountain, Latin could use in montem.

Here olim means something like:

• once
• long ago
• formerly

So it tells us that the seeing happened at some time in the past.

In different contexts, olim can have slightly different shades of meaning, but here once / long ago is the natural sense.

The sentence breaks down like this:

• Avus = subject of the main clause
• meminit = main verb
• se olim lupum ferocem in monte vidisse = indirect statement

Inside that indirect statement:

• se = subject of vidisse
• olim = adverb modifying the action of seeing
• lupum ferocem = direct object
• in monte = place where the wolf was seen
• vidisse = perfect infinitive

So the core pattern is:

Avus meminit + accusative + infinitive

That is a very useful pattern to recognize in Latin.

Latin word order is much more flexible than English word order because Latin uses case endings to show grammatical relationships.

So even if the words move around, the endings still tell you what each word is doing.

In this sentence:

• Avus is clearly the subject because it is nominative
• lupum is clearly the object because it is accusative
• monte is ablative after in
• vidisse naturally comes at the end of the indirect statement

A very literal order would be something like:

Grandfather remembers himself once a fierce wolf on the mountain to have seen

That sounds odd in English, but it is normal Latin structure.

Yes, but the meaning would change.

• se refers back to the main subject, avus
• eum would refer to someone else, not the grandfather

So:

• Avus meminit se ... vidisse = Grandfather remembers that he himself saw...
• Avus meminit eum ... vidisse = Grandfather remembers that that man / he saw..., where he is someone other than the grandfather

So se is important because it shows that the rememberer and the seer are the same person.