يحل صديقي اللغز ويفك الشفرة.

This is a very common Arabic sentence pattern: verb + subject + object.

So in يحل صديقي اللغز:

• يحل = the verb
• صديقي = the subject
• اللغز = the object

This is called a verbal sentence in Arabic.

English usually prefers subject + verb + object:

• My friend solves the riddle

Arabic can also use subject-first order:

• صديقي يحل اللغز ويفك الشفرة

Both are possible, but the original sentence uses a very natural verb-first structure.

Yes. That is also correct.

The difference is mostly about style and emphasis:

• يحل صديقي اللغز ويفك الشفرة = more classically verbal, often very natural in MSA
• صديقي يحل اللغز ويفك الشفرة = starts by emphasizing my friend

So both mean the same thing, but the word order changes what feels slightly more prominent.

صديقي means my friend.

It is made of:

• صديق = friend
• ـي = my

So:

• صديق = a friend / friend
• صديقي = my friend

That final ـي is a possessive suffix attached directly to the noun.

Because the verb itself already tells you the subject is third person masculine singular.

يحل means he solves / he is solving depending on context.

In Arabic, subject pronouns are often built into the verb, so you do not need a separate word like هو unless you want extra emphasis.

So:

• يحل already contains the idea of he solves
• then صديقي tells you who that he is

Arabic often avoids repeating the subject when it is clearly the same.

So in:

• يحل صديقي اللغز ويفك الشفرة

the subject صديقي applies to both verbs:

• يحل ... = he solves ...
• ويفك ... = and he deciphers / cracks ...

Arabic does not need to repeat صديقي a second time unless you want special emphasis or clarification.

They are in the imperfect form, which often corresponds to the present in English.

Depending on context, the imperfect can mean:

• present/habitual: he solves, he deciphers
• ongoing: he is solving, he is deciphering
• sometimes even future if the context shows that

In a simple sentence like this, the most natural understanding is usually:

• he solves the riddle and deciphers the code or
• he is solving the riddle and deciphering the code

Because Arabic often uses different verbs depending on the kind of thing being solved.

Here the combinations are very natural:

• يحل اللغز = solve the riddle/puzzle
• يفك الشفرة = crack/decipher the code

The two verbs are related in meaning, but they are not identical:

• حلّ often means solve
• فكّ literally has the sense of undo, unfasten, take apart, and by extension decode / crack

So this sentence sounds idiomatic because each verb matches its object well.

Because they are definite nouns:

• اللغز = the riddle
• الشفرة = the code

If you removed الـ, the meaning would become indefinite:

• لغز = a riddle
• شفرة = a code

So the sentence as written refers to specific things: the riddle and the code.

Yes, in fully vocalized writing, both verbs normally have a shadda on the last consonant:

• يَحُلُّ
• يَفُكُّ

That is because both come from verbs with a doubled final consonant:

• حلّ
• فكّ

In normal everyday Arabic writing, short vowels and other diacritics are usually omitted, so you often see:

• يحل
• يفك

even though the pronunciation still includes the doubled consonant.

In full grammatical vocalization:

• the subject is nominative
• the direct objects are accusative

So you would typically have:

• صديقي = subject
• اللغزَ = object, accusative
• الشفرةَ = object, accusative

And the verbs could appear as:

• يَحُلُّ
• يَفُكُّ

A fully vocalized version would look like this:

• يَحُلُّ صَدِيقِي اللُّغْزَ وَيَفُكُّ الشِّفْرَةَ

In normal printed Arabic, these endings are usually not written.

The pronunciation changes because of sun letters.

• الشفرة: the letter ش is a sun letter, so الـ is pronounced with assimilation:
  ash-shifra(h)

• اللغز: the word begins with ل, which also causes assimilation. You hear a doubled l sound:
  al-lughz

So even though both words are written with الـ, the pronunciation is not always a plain al-.

و means and.

It connects the two actions:

• يحل صديقي اللغز = my friend solves the riddle
• ويفك الشفرة = and deciphers the code

It is attached directly to the next word in Arabic writing, so ويفك is literally:

• و = and
• يفك = he deciphers / cracks

It describes two coordinated actions done by the same subject:

1. يحل اللغز = solves the riddle
2. يفك الشفرة = deciphers the code

So the sentence presents my friend as doing both things. Arabic often links actions this way very smoothly with و.