تشمل الأحجية رمزا سريا.

This sentence uses a very common Arabic pattern:

verb + subject + object

• تشمل = the verb, includes
• الأحجية = the subject, the puzzle
• رمزا سريا = the object, a secret symbol/code

So Arabic often begins the sentence with the verb, unlike English, which usually starts with the subject.

Because the subject الأحجية is a singular feminine noun.

• الأحجية is grammatically feminine
• so the verb must be feminine too
• تشمل is the 3rd person feminine singular form: it includes

In full vocalization, it would be تَشْمَلُ.

تشمل is the imperfect/present form of the verb شَمِلَ or شَمَلَ in usage meaning to include / encompass / cover.

Here it means something like:

• includes
• contains
• encompasses

So تشمل الأحجية... literally means the puzzle includes...

Because الأحجية is definite, while رمزا سريا is indefinite.

• الأحجية = the puzzle
• رمز = a symbol/code
• سري = secret

Arabic has a definite article, الـ, but it does not have a separate word for a/an.
Instead, a noun is usually understood as indefinite simply by not having الـ.

So:

• الأحجية = the puzzle
• رمز = a symbol

Because they are in the accusative case, and the phrase is indefinite.

In full vocalization, you would write:

• رَمْزًا
• سِرِّيًّا

Why accusative?

Because رمزا سريا is the direct object of the verb تشمل.

And why do both words match?

Because سريا is an adjective describing رمزا, and Arabic adjectives agree with the nouns they describe in:

• case
• definiteness
• gender
• number

So both are:

• masculine
• singular
• indefinite
• accusative

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

So:

• رمز سري = a secret symbol/code
• literally: symbol secret

This is the normal Arabic order, unlike English, where the adjective usually comes first.

Because Arabic normally does not use an indefinite article like English a/an.

Instead:

• رمز means a symbol or simply symbol in an indefinite sense
• الرمز means the symbol

So indefiniteness is shown by:

• the absence of الـ
• and, in fully vocalized Arabic, often by tanwīn (the -n sound), as in رَمْزًا

سري means secret.

It is an adjective describing رمز.
So رمز سري means:

• secret symbol
• secret code
• sometimes even password/code depending on context

The exact English wording can vary a little by context, but grammatically it is simply noun + adjective.

In pause, it is usually pronounced roughly as:

al-uḥjiyya

If you pronounce the full case ending in careful Modern Standard Arabic, it would be:

al-uḥjiyyatu

A few useful points:

• the ة at the end is taa marbuuṭa
• in pause, it usually sounds like -a / -ah
• when full endings are pronounced, it becomes -tu / -ta / -ti depending on case

So in this sentence, with full case ending, it would be الأحجيةُ because it is the subject.

Yes. Arabic allows that too.

You could say:

الأحجية تشمل رمزا سريا.

That is more like The puzzle includes a secret symbol in subject-first order.

Both are correct, but:

• تشمل الأحجية... is more classically verbal in style
• الأحجية تشمل... is more directly parallel to English word order

The meaning stays basically the same.