صديقي فقط يخلط رزمة البطاقات.

Because صديقي is صديق + the possessive suffix -ي, which means my.

So:

• صديق = friend
• صديقي = my friend

Arabic often expresses possession this way instead of using a separate word for my.

Arabic can use both subject-first and verb-first word order.

So both of these are possible:

• صديقي فقط يخلط رزمة البطاقات
• يخلط صديقي رزمة البطاقات

Starting with صديقي gives the subject a little more prominence, similar to putting extra stress on my friend in English.

فقط means only or just.

A very important point is that فقط often relates to the word or phrase near it. In this sentence, because it comes right after صديقي, many learners will read it as emphasizing my friend.

For example, compare:

• صديقي فقط يخلط رزمة البطاقات = Only my friend shuffles the deck of cards / My friend alone shuffles the deck of cards
• صديقي يخلط رزمة البطاقات فقط = My friend shuffles only the deck of cards

In real use, context helps decide exactly what only is limiting.

يخلط is the imperfect (non-past) form of the verb خلط.

Here it is:

• خلط = to mix / shuffle
• يخلط = he mixes, he is mixing, or he shuffles

It is third person masculine singular, matching صديقي.

It can mean either, depending on context.

The Arabic imperfect often covers several English meanings, such as:

• he shuffles
• he is shuffling
• sometimes even he will shuffle

In an isolated example like this, English could translate it as either shuffles or is shuffling, depending on what meaning was intended.

Because the subject صديقي is singular masculine.

So Arabic uses a singular masculine verb form:

• صديقي يخلط = my male friend shuffles

If the subject were feminine, you would normally say:

• صديقتي تخلط = my female friend shuffles

It is an iḍāfa construction, often called a genitive construction or construct phrase.

That means two nouns are joined together:

• رزمة = pack / bundle / deck
• البطاقات = the cards

Together:

• رزمة البطاقات = the deck/pack of cards

This structure is very common in Arabic.

Because in an iḍāfa construction, the first noun normally does not take ال.

So Arabic says:

• رزمة البطاقات

not:

• الرزمة البطاقات

Even though رزمة has no ال, the whole phrase can still be definite because the second noun is definite.

So:

• رزمة البطاقات = the deck of cards
• رزمة بطاقات = a deck of cards

Because it is the second part of the iḍāfa, and it is definite.

In Arabic, the definiteness of the whole iḍāfa phrase is usually determined by the second noun. Since البطاقات is definite, رزمة البطاقات is also definite.

Also, بطاقات is the plural of بطاقة.

Yes. In normal Arabic writing, short vowels and many case endings are usually not written.

A fully vocalized version could look like this:

• صديقي فقط يخلطُ رزمةَ البطاقاتِ

Here:

• يخلطُ has a final ـُ
• رزمةَ is in the accusative because it is the direct object
• البطاقاتِ is in the genitive because it is the second noun in the iḍāfa

In everyday printed Arabic, these endings are usually omitted.

It is understandable, and it does mean cards, but it can be a bit general.

If you want to be more specific about playing cards, Arabic often uses:

• أوراق اللعب = playing cards

So another natural version could be:

• صديقي فقط يخلط رزمة أوراق اللعب

That said, رزمة البطاقات is still easy to understand.