Der Fahrlehrer erklärt jedes Verkehrszeichen so deutlich, dass sogar mein Bruder es sofort versteht.

Der Fahrlehrer is the subject of the main clause, so it is in the nominative case.

• der = masculine nominative singular
• Fahrlehrer = driving instructor / driving teacher

The verb erklärt agrees with this subject:

• Der Fahrlehrer erklärt ... = The driving instructor explains ...

If the noun were in a different role, the article might change:

• Ich sehe den Fahrlehrer. = I see the driving instructor.
  Here it becomes den because it is accusative.

Fahrlehrer is a compound noun:

• fahren = to drive
• Lehrer = teacher

So Fahrlehrer literally means something like driving teacher, but the natural English translation is driving instructor.

German uses compound nouns very often, so learners should get used to breaking them apart:

• Verkehrszeichen = traffic sign
• Fahrschule = driving school
• Führerschein = driver's license

Jedes Verkehrszeichen means each traffic sign or every traffic sign. It treats the signs one by one as singular items.

• jedes Verkehrszeichen = each/every traffic sign
• alle Verkehrszeichen = all traffic signs

Both are possible in German, but they are slightly different in focus:

• Der Fahrlehrer erklärt jedes Verkehrszeichen ...
  He explains each individual sign ...
• Der Fahrlehrer erklärt alle Verkehrszeichen ...
  He explains all the signs ...

So jedes gives a more individual, item-by-item feeling.

Because Verkehrszeichen is a neuter singular noun here, and jedes has to match it.

German jed- works like an article-type word and changes its ending depending on gender, number, and case.

Here:

• Verkehrszeichen is neuter
• it is the direct object of erklärt, so it is accusative
• neuter nominative and accusative singular take -es

So:

• jedes Verkehrszeichen

Compare:

• jeder Mann = every man
• jede Frau = every woman
• jedes Kind = every child

It is in the accusative case, because it is the direct object of erklären.

Ask the question:

• Der Fahrlehrer erklärt was?
  What does the driving instructor explain?
  Jedes Verkehrszeichen.

That makes it the direct object.

A helpful detail: Verkehrszeichen is one of those neuter nouns whose form often looks the same in nominative and accusative, so the article/determiner (jedes) gives you the main clue.

Because deutlich is being used adverbially here. It describes how the driving instructor explains something.

• so deutlich erklären = to explain so clearly

In German, adjectives used as adverbs usually do not change their form:

• Er spricht schnell. = He speaks quickly.
• Sie singt schön. = She sings beautifully.
• Er erklärt deutlich. = He explains clearly.

But when an adjective comes before a noun, it does take an ending:

• ein deutliches Zeichen = a clear sign
• eine deutliche Erklärung = a clear explanation

So ... dass is a very common structure meaning so ... that.

In your sentence:

• so deutlich = so clearly
• dass sogar mein Bruder es sofort versteht = that even my brother understands it immediately

So the whole structure is:

• Der Fahrlehrer erklärt jedes Verkehrszeichen so deutlich, dass ...
• The driving instructor explains every traffic sign so clearly that ...

Other examples:

• Er ist so müde, dass er sofort einschläft.
  He is so tired that he falls asleep immediately.
• Sie spricht so leise, dass niemand sie hört.
  She speaks so quietly that nobody hears her.

Because dass introduces a subordinate clause, and in German subordinate clauses are separated from the main clause by a comma.

So:

• Der Fahrlehrer erklärt jedes Verkehrszeichen so deutlich, dass ...

This is standard German punctuation.

A very useful rule:

• If you see dass, expect:
  1. a comma before it
  2. the conjugated verb to go to the end of that clause

Because the clause beginning with dass is a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

So:

• ..., dass sogar mein Bruder es sofort versteht.

If it were a main clause, the normal order would be:

• Sogar mein Bruder versteht es sofort.

But after dass, German pushes the verb to the end:

• ..., dass sogar mein Bruder es sofort versteht.

This is one of the most important word-order rules in German.

Es refers back to jedes Verkehrszeichen.

So the idea is:

• The driving instructor explains each traffic sign so clearly
• that even my brother understands it immediately

Why es? Because Verkehrszeichen is neuter singular, and the pronoun for a neuter singular noun in the accusative is es.

Compare:

• der Tisch → ihn
• die Lampe → sie
• das Buch → es

Since das Verkehrszeichen is neuter, the pronoun is es.

German, like English, usually uses a pronoun to avoid repeating the same noun.

So instead of saying:

• ..., dass sogar mein Bruder jedes Verkehrszeichen sofort versteht

German uses the shorter and more natural:

• ..., dass sogar mein Bruder es sofort versteht

Both are grammatically possible in some contexts, but the pronoun sounds smoother here because the noun has just been mentioned.

Sogar means even.

It adds emphasis and shows that the speaker thinks my brother is perhaps not someone who would normally understand quickly.

So:

• sogar mein Bruder = even my brother

It suggests something like:

• if even my brother understands it, then the explanation must be very clear

Other examples:

• Sogar Kinder verstehen das. = Even children understand that.
• Er hat sogar am Sonntag gearbeitet. = He even worked on Sunday.

German often places the focused or emphasized element early in the clause. Here, sogar mein Bruder is the important surprising element.

The subordinate clause is:

• dass sogar mein Bruder es sofort versteht

This order is natural because:

• sogar mein Bruder is the subject
• es is the object
• sofort is an adverb
• versteht goes at the end because of the subordinate clause

A main-clause version would be:

• Sogar mein Bruder versteht es sofort.

Sofort means immediately or right away.

Here it comes before the final verb:

• ..., dass sogar mein Bruder es sofort versteht.

That is very natural in German subordinate clauses.

You may also see it in main clauses:

• Mein Bruder versteht es sofort.
• Sofort versteht mein Bruder es.
  This is possible, but it gives stronger emphasis to sofort.

So the placement can vary, but in your sentence the position is normal and neutral.

Here it is best translated as clearly, because it describes the verb erklärt.

• deutlich as an adjective: clear
  • ein deutliches Zeichen = a clear sign
• deutlich as an adverb: clearly
  • etwas deutlich erklären = to explain something clearly

So in this sentence:

• so deutlich = so clearly

No, not in standard German. The verb verstehen normally needs an object here: someone understands something.

So you need:

• ..., dass sogar mein Bruder es sofort versteht.

Without es, the sentence would feel incomplete because the thing being understood has to be expressed somehow.

Because mein Bruder is the subject of the subordinate clause.

In the second clause, ask:

• Wer versteht es sofort?
  Who understands it immediately?
  mein Bruder

That makes it nominative.

So the sentence has:

• Main clause subject: Der Fahrlehrer
• Subordinate clause subject: mein Bruder

Yes, that is a very good way to understand it.

Main clause:

• Der Fahrlehrer erklärt jedes Verkehrszeichen so deutlich

Subordinate clause:

• dass sogar mein Bruder es sofort versteht

The first clause sets up the degree:

• so deutlich = so clearly

The second clause gives the result:

• dass ... versteht = that ... understands it

So the overall pattern is:

• [main clause] + so ... , dass + [subordinate clause]

That pattern is extremely common in German.