Mane nebula tam magna erat ut mons prope villam non videretur.

Mane is an adverb meaning in the morning or early in the morning. It sets the time for the whole sentence right away.

Latin often puts time expressions near the beginning, so starting with mane is very natural.

The subject of the main clause is nebula, meaning fog or mist.

It is nominative singular, and it matches erat:

• nebula = the fog
• erat = was

So the main clause is built around nebula ... erat.

Tam means so, and magna means great/large, here in the sense of thick/dense.

Together, tam magna means so great or so thick.

This is a very common Latin pattern:

• tam ... ut ... = so ... that ...

So tam magna erat ut ... means it was so thick that ...

Also, magna agrees with nebula:

• feminine singular nominative nebula
• feminine singular nominative magna

Here ut introduces a result clause.

The pattern is:

• tam = so
• ut = that

So:

• tam magna erat ut mons non videretur
• the fog was so thick that the mountain could not be seen

This is different from some other uses of ut, such as purpose clauses. Here it is clearly a result, because tam signals the consequence.

Because it is in a result clause introduced by ut.

In Latin, result clauses normally take the subjunctive. Since the main verb is past (erat), Latin uses the imperfect subjunctive here:

• main verb: erat
• result clause verb: videretur

So the subjunctive is not random; it is required by the grammar of the tam ... ut ... construction.

Videbatur would be was being seen or seemed in an ordinary indicative statement.

But after tam ... ut, Latin needs a subjunctive verb for the result clause, so it becomes videretur.

Compare:

• videbatur = indicative
• videretur = imperfect subjunctive

So videretur is used because of the clause type, not because the meaning itself changes drastically.

Because mons is the thing being seen.

• videre = to see
• videri = to be seen

So:

• mons non videretur = the mountain was not seen / the mountain could not be seen / the mountain was not visible

In English, we often translate this more naturally as could not be seen, even though the Latin form is simply passive.

Because mons is the subject of videretur.

Even though it comes after ut, it is still the subject of that clause:

• ut mons ... non videretur
• that the mountain ... could not be seen

So mons is nominative singular, not accusative.

Because prope is a preposition that takes the accusative.

So:

• villa = nominative/ablative
• villam = accusative

Therefore:

• prope villam = near the villa/farmhouse

A learner should also remember that Latin villa usually means a country house, estate, or farmhouse, not exactly the same thing as the modern English word villa.

It describes mons:

• mons prope villam = the mountain near the villa

So the mountain is the thing near the villa, not the fog.

Non negates the verb, so non videretur means was not seen or could not be seen.

Its position is very natural in Latin, but Latin word order is flexible. The important thing is that non belongs with the verb of the result clause.

The imperfect often gives background description in past time.

Here the sentence is painting a scene:

• In the morning, the fog was so thick...

That kind of continuing background condition is exactly what the imperfect is good at.

If you used fuit, it would sound more like a completed fact:

• there was fog rather than
• the fog was hanging there / the scene was like this

Yes. That would also be good Latin.

Two common ways to express this idea are:

• tam magna nebula erat ut...
• tanta nebula erat ut...

Both mean something like the fog was so great/thick that...

The version in your sentence uses tam + adjective:

• tam magna

Another common pattern uses tantus agreeing directly with the noun:

• tanta nebula

Not really. It is quite natural Latin.

The sentence is arranged in a very readable way:

• Mane = time first
• nebula tam magna erat = main statement
• ut... non videretur = result saved for the end

Latin often places the verb at or near the end of a clause, so ending with videretur is very typical. The order also builds suspense nicely: first the fog, then how thick it was, then the result.