الأحجية صعبة بسبب الرمز السري.

In Arabic, the verb to be in the present tense is usually not written or said.

So:

• الأحجية صعبة literally looks like the riddle difficult
• but it means The riddle is difficult

This is a very normal Arabic sentence pattern, often called a nominal sentence.

If you wanted was, will be, etc., Arabic would usually use an actual verb.

Because الأحجية is a feminine noun, the adjective describing it must also be feminine.

• الأحجية = the riddle / puzzle
• صعبة = difficult (feminine form)

Arabic adjectives must agree with the noun in things like:

• gender
• number
• definiteness

The masculine form would be صعب.

A big clue is the ending ـة (called tāʾ marbūṭa), which often marks feminine nouns.

So:

• أحجية ends in ـة
• that strongly suggests it is feminine

Not every feminine noun ends this way, and not every noun with this kind of ending behaves exactly the same, but it is one of the most common signs of feminine gender in Arabic.

In Arabic, adjectives normally come after the noun they describe.

So:

• أحجية صعبة = a difficult riddle
• الأحجية صعبة = the riddle is difficult

This is the normal word order for noun + adjective in Arabic.

بسبب means because of or due to.

In this sentence:

• بسبب الرمز السري = because of the secret symbol/code

It introduces the reason that the riddle is difficult.

A useful way to think of it:

• بـ = with / by / because of
• سبب = reason / cause

Together, بسبب functions like the preposition because of.

Because بسبب is followed by the thing that causes the situation.

So the structure is:

• بسبب + noun
• because of + noun

Here:

• بسبب الرمز السري = because of the secret symbol

Also, nouns after prepositions in Arabic are normally in the genitive case. In fully vowelled Arabic, that would affect the ending, though in ordinary writing those short vowel endings are usually not shown.

Because in Arabic, the adjective usually comes after the noun.

So:

• الرمز = the symbol
• السري = the secret

Together:

• الرمز السري = the secret symbol

Also, the adjective must match the noun in definiteness, so both have الـ:

• الرمز = the symbol
• السري = the secret

This matching is required in normal adjective-noun combinations.

Because Arabic adjectives agree with the nouns they describe in definiteness.

Since the noun is definite:

• الرمز = the symbol

the adjective must also be definite:

• السري = the secret

So:

• الرمز السري literally = the symbol the-secret
• natural English = the secret symbol

If the noun were indefinite, the adjective would also be indefinite:

• رمز سري = a secret symbol

A common pronunciation is approximately:

• al-uḥjiyya

Some notes:

• ح is a strong breathy sound from the throat, not like English h
• ج in Modern Standard Arabic is often taught as j as in judge
• the ending ـية here gives something like -iyya

So the full sentence is roughly:

• al-uḥjiyyatu ṣaʿbatun bisababi ar-ramzi as-sirriyyi in fully vocalized formal reading

But in normal speech or unvowelled text, many final case endings are not pronounced clearly.

This is because of sun letters and moon letters.

• In الرمز, the letter ر is a sun letter
• In السري, the letter س is also a sun letter

With sun letters, the l sound of ال is not pronounced, and the next consonant is doubled.

So these are pronounced roughly like:

• ar-ramz
• as-sirrī

But in writing, the spelling still stays الرمز and السري.

Yes, but the structure would change slightly.

Your sentence uses:

• بسبب الرمز السري = because of the secret symbol

If you use لأن (because), you usually follow it with a whole clause, not just a noun phrase. For example:

• الأحجية صعبة لأن الرمز السري معقد
• The riddle is difficult because the secret symbol is complicated

So:

• بسبب = because of
  • noun
• لأن = because
  • full sentence/clause

A simple breakdown is:

• الأحجية = the subject/topic
• صعبة = the description/predicate
• بسبب الرمز السري = a prepositional phrase giving the reason

So the sentence structure is:

• noun + adjective/predicate + reason phrase

More literally:

• The riddle [is] difficult because of the secret symbol.

Yes. In formal grammar, Arabic words in this sentence do have case endings, but they are usually not written in normal everyday text.

In fully vocalized MSA, you might see something like:

• الأُحْجِيَةُ صَعْبَةٌ بِسَبَبِ الرَّمْزِ السِّرِّيِّ

Very roughly:

• الأحجيةُ = nominative
• صعبةٌ = nominative
• بسببِ = preposition
• الرمزِ = genitive after the preposition
• السريِّ = genitive to match الرمز

For learners, it is completely normal at first to read the sentence without focusing on those endings unless you are studying formal grammar or fully vocalized texts.