Tantus imber cecidit ut via ad forum paene flumini similis videretur.

This is a very common Latin pattern for a result clause.

• tantus = so great, such a great
• ut = that

So tantus imber cecidit ut ... means so great a rain fell that ... or more naturally in English, so much rain fell that ...

The idea is:

• first, Latin states the degree: tantus imber
• then it states the result: ut via ... videretur

So the whole structure is so/such ... that ...

Because tam and tantus do different jobs.

• tam usually modifies an adjective or adverb:
  
  • tam magnus = so big
  • tam celeriter = so quickly
• tantus is an adjective meaning so great or such, and it can directly modify a noun:
  • tantus imber = so great a rain / such a heavy rain

Since imber is a noun, tantus is the natural choice.

Cecidit is the perfect tense of cadere, meaning to fall.

So imber cecidit literally means rain fell.

This is very natural in Latin, and it is not strange in English either—we can also say rain fell. Latin often describes weather with this kind of imagery: the rain falls.

So:

• imber = rainstorm / rain
• cecĭdit = fell

Together: a heavy rain fell or there was such a heavy downpour

Because it is inside a result clause introduced by ut after tantus.

In Latin, result clauses normally use the subjunctive:

• tantus ... ut videretur = so great ... that it seemed ...

If this were a simple independent statement, Latin would probably use the indicative:

• via ... videbatur = the road seemed ...

But after tantus ... ut, Latin switches to the subjunctive:

• via ... videretur

This is because of sequence of tenses.

The main verb is cecĭdit, a past tense. After a past-tense main verb, Latin normally uses a secondary sequence in subordinate clauses.

Then Latin chooses the subjunctive tense based on time relationship:

• imperfect subjunctive = action/state happening at the same time as the main past action
• pluperfect subjunctive = action/state happening before the main past action

Here, the road seemed river-like at the time the rain had fallen / as a result of that rain. So Latin uses the imperfect subjunctive:

• videretur = seemed

The subject is via.

So the result clause is built around:

• via ... videretur = the road seemed ...

The phrase ad forum goes with via and describes which road:

• via ad forum = the road to the forum

So the subject is not imber and not forum. It is via.

Because similis normally takes the dative in Latin.

So:

• similis flumini = similar to a river

This is something English speakers simply have to get used to, because English uses to + noun, while Latin often uses the dative directly after certain adjectives.

In this sentence:

• similis agrees with via
• flumini is the dative complement after similis

So literally:

• via ... flumini similis = the road ... similar to a river

Because similis is agreeing with via, which is the subject and is nominative singular feminine.

• via = nominative singular feminine
• similis = nominative singular masculine/feminine

This is a predicate adjective, like in English the road seemed similar.

So the grammar is:

• via = subject
• similis = adjective describing the subject
• flumini = dative after similis

It is not similem because that would be accusative, and nothing here calls for an accusative adjective.

It means the road to the forum.

• via = road, street
• ad forum = to the forum

Here ad shows direction or destination. The phrase identifies the road by where it leads.

So via ad forum is not the road at the forum. It is the road leading to the forum.

Depending on context, forum can mean the marketplace, public square, or town center.

Paene means almost.

It modifies the idea of resemblance:

• paene flumini similis = almost like a river
• more literally, almost similar to a river

So Latin is saying the road did not literally become a river, but it looked nearly like one.

Yes—mainly that Latin word order is flexible, but the sentence is arranged in a very typical and elegant way.

Structure:

• Tantus imber cecidit = main clause
• ut via ad forum paene flumini similis videretur = result clause

A few things to notice:

• tantus comes early for emphasis: so great
• ut clearly signals the result clause
• videretur comes at the end, which is very common in Latin subordinate clauses

A fairly literal order would be:

• So great a rain fell that the road to the forum almost similar to a river seemed.

That is awkward English, but it helps show how Latin builds the sentence.

Yes:

• Tantus — so great / such a great; adjective, nominative singular masculine, agreeing with imber
• imber — rain / rainstorm; noun, nominative singular masculine; subject of cecĭdit
• cecĭdit — fell; perfect active indicative, 3rd singular, from cadere
• ut — that; introduces the result clause
• via — road / street; nominative singular feminine; subject of videretur
• ad — to
• forum — forum / marketplace; accusative singular after ad
• paene — almost; adverb
• flumini — to a river / than a river / like a river; dative singular of flumen, used with similis
• similis — similar / like; nominative singular feminine, agreeing with via
• videretur — seemed; imperfect subjunctive, 3rd singular, from videri

So the core grammar is:

• Tantus imber cecidit = Such heavy rain fell
• ut via ... videretur = that the road ... seemed
• flumini similis = like a river