Avus meminit se olim multa signa tempestatis in caelo observare solitum esse.

Because memini, meminisse is a special verb: its perfect forms have a present meaning.

So:

• meminit = he/she remembers
• meminerat = he/she remembered / had remembered
• meminisse = to remember

Historically it is a perfect form, but in normal translation you usually treat meminit as present: Grandfather remembers.

Se is the reflexive pronoun, and it refers back to the subject of the main clause, avus.

So:

• avus meminit se... = Grandfather remembers that he...
• the he is the same person as grandfather

If Latin used eum instead, that would normally mean some other man, not the grandfather himself.

Because this sentence uses an indirect statement construction, often called the accusative-and-infinitive construction.

After verbs like meminit, Latin can say:

• meminit se observare solitum esse

Literally, this is something like:

• he remembers himself to have been accustomed to observe

In better English:

• he remembers that he used to observe

In this construction:

• the subject of the reported statement goes into the accusative: se
• the verb of the reported statement goes into the infinitive

Because two different things are happening at once.

1. soleo normally takes an infinitive:

   • soleo observare = I am accustomed to observe / I usually observe

2. In indirect statement, Latin uses an infinitive for the reported verb:

   • se ... solitum esse

So the full idea is:

• se observare solitum esse
• that he had been accustomed to observe
• more naturally: that he used to observe

Here:

• observare depends on solitum esse
• solitum esse is the infinitive phrase required by indirect statement

Solitum esse comes from soleo, solere, solitus sum, which means to be accustomed, to be in the habit of, or more simply to used to.

So:

• observare solitum esse = to have been accustomed to observe
• natural English: to have used to observe or better simply to have used to watch for / to have been in the habit of observing

In this sentence, it expresses a habitual past action.

Because solitum agrees with se, and se is accusative singular masculine here.

In an accusative-and-infinitive construction, a participle or adjective referring to the subject of the infinitive also goes into the accusative.

So:

• se = accusative singular masculine
• solitum = accusative singular masculine

If the subject were feminine, you would expect solitam instead.

This is the object of observare.

Break it down:

• multa = many
• signa = signs
• tempestatis = of a storm / of weather

So:

• multa signa tempestatis = many signs of a storm or many weather-signs, depending on context

Grammatically:

• multa signa is accusative plural
• tempestatis is genitive singular, depending on signa

Because it depends on signa.

Latin often uses the genitive to show a relationship like of ...:

• signa tempestatis = signs of a storm
• signs belonging to / indicating a storm

So tempestatis is a dependent genitive telling us what kind of signs these are.

Olim usually means once, formerly, in former times, or long ago.

In this sentence it marks the habit as belonging to an earlier time:

• se olim ... observare solitum esse
• that he once / formerly used to observe...

It does not necessarily mean one single occasion; here it helps locate the habit in the past.

Because in with the ablative often expresses location:

• in caelo = in the sky

If motion into something were meant, Latin would typically use in with the accusative instead.

So here the idea is location, not movement:

• the signs were being observed in the sky

The sentence breaks down like this:

• Avus = subject of the main verb
• meminit = main verb
• se olim multa signa tempestatis in caelo observare solitum esse = indirect statement

Inside the indirect statement:

• se = subject of the infinitive clause
• observare = complementary infinitive with solitum esse
• solitum esse = the core infinitive idea, to have been accustomed
• multa signa tempestatis = object of observare
• in caelo = where the signs were observed
• olim = when this habit belonged

So the sentence is built as:

• Grandfather remembers [that he formerly used to observe many signs of a storm in the sky].

Because Latin often prefers the accusative-and-infinitive construction after verbs of saying, thinking, perceiving, and remembering.

English says:

• He remembers that he used to observe...

Latin often says:

• He remembers himself to have been accustomed to observe...

That sounds strange in English, but it is a very normal Latin pattern.

It may feel unusual from an English point of view, but it is very normal in Latin.

Latin word order is more flexible because endings show grammatical relationships. Writers often place words for emphasis, rhythm, or clarity.

Here, the important main verb comes early:

• Avus meminit

Then the remembered idea follows, with solitum esse at the end, which neatly closes the indirect statement.

A learner should focus first on the grammar, not on forcing the words into English order one by one.