Avia meminit se, cum iuvenis esset, saepe ante lararium stetisse et rosas ibi posuisse.

Because meminit comes from memini, a verb that is perfect in form but present in meaning. So:

• meminit = he/she remembers
• meminerat = he/she remembered / had remembered

This is a common special feature of memini.

Latin often uses an accusative + infinitive construction where English uses a that-clause.

So instead of saying:

• Grandma remembers that she stood...

Latin says:

• Avia meminit se ... stetisse...

Literally, this is something like:

• Grandma remembers herself to have stood...

But in natural English, we translate it as Grandma remembers that she stood...

Se is the reflexive pronoun, meaning herself here. It refers back to avia.

In an accusative + infinitive construction, the subject of the infinitive goes into the accusative, so se is the accusative subject of stetisse and posuisse.

So:

• avia meminit se stetisse = grandmother remembers that she stood

Because se is accusative, while sibi is dative.

Here the pronoun is functioning as the subject of the infinitives stetisse and posuisse, and in this construction that subject must be accusative.

So:

• se = correct
• sibi = would be the wrong case here

Because they are part of the indirect statement after meminit.

After verbs like say, think, know, remember, Latin very often expresses the reported idea with:

• an accusative subject
• plus an infinitive verb

Here:

• se = subject
• stetisse and posuisse = infinitives expressing what she remembers doing

The perfect infinitive shows action that happened before the time of meminit.

She remembers now, but the standing and placing happened earlier, so Latin uses perfect infinitives:

• stetisse = to have stood
• posuisse = to have placed

In smoother English, we often just say stood and placed rather than to have stood and to have placed.

It is the perfect infinitive of sto, stare, steti.

The perfect stem is stet-, and adding -isse gives:

• stetisse

So the parts are:

• sto = I stand
• steti = I stood / have stood
• stetisse = to have stood

Many Latin verbs change stem in the perfect, so this is normal even if it feels unfamiliar at first.

Because this is a past cum clause giving background circumstance: when she was young.

In that kind of clause, Latin often uses the subjunctive, especially in past narrative.

So:

• cum ... esset = when she was young

The imperfect subjunctive fits the idea of an ongoing past state.

It is part of what she remembers.

The sense is:

• Grandma remembers that, when she was young, she often stood before the lararium and placed roses there

Not:

• Grandma, when she was young, remembers...

So cum iuvenis esset gives the time of the remembered actions, not the time of meminit.

Because it goes with esset, a form of esse. After esse, Latin uses a predicate nominative.

So:

• iuvenis esset = she was young or she was a young woman

Also, iuvenis has the same form for masculine and feminine singular, so it can describe avia without changing form.

For stetisse and posuisse, the subject is the expressed se.

For esset, the subject is understood as she, referring to the same person, avia.

A useful distinction is this:

• the infinitives take se as their accusative subject
• the finite verb esset has its own understood subject in the normal way

So the whole sentence still refers to the grandmother throughout.

Because ante takes the accusative.

So:

• ante lararium = before the lararium

This is true even though there is no movement here. English before does not show case, but Latin ante regularly governs the accusative.

Ibi means there, referring back to the place just mentioned.

So ante lararium tells you the location, and ibi reinforces it:

• she stood before the shrine and placed roses there

In English this can sound a little redundant, but Latin often likes this kind of clarifying adverb.

Latin word order is flexible, so adverbs like saepe can be placed in different spots for emphasis or rhythm.

Here saepe naturally goes with the remembered actions:

• se ... saepe ... stetisse et ... posuisse

So the sense is that she often did these things.

Even if the words moved around, the grammar would still show what belongs together.

Because one accusative subject can serve for both coordinated infinitives.

So:

• se ... stetisse et ... posuisse

means:

• that she had stood and had placed

Latin does not need to repeat se unless it wants extra emphasis or clarity.