يشفر صديقي النسخة الاحتياطية.

Because Arabic often uses verb–subject–object order in a neutral verbal sentence.

So:

• يشفر صديقي النسخة الاحتياطية = encrypts my friend the backup copy literally = My friend encrypts the backup copy naturally in English

You can also say:

• صديقي يشفر النسخة الاحتياطية

That is also correct, but it gives صديقي a bit more prominence, like as for my friend, he encrypts the backup copy.

Here it is most naturally understood as يُشَفِّرُ (yushaffiru), from the verb شَفَّرَ.

This is Form II, which often has a causative or intensive sense. In modern usage, شَفَّرَ commonly means to encrypt / encode.

A few useful points:

• The shadda on ف means that consonant is doubled.
• Arabic normally writes this word without short vowels, so يشفر could look ambiguous by itself.
• Context tells you the intended reading here.

It can mean either, depending on context.

The Arabic present/imperfect often covers:

• simple present: he encrypts
• habitual: he usually encrypts
• present progressive: he is encrypting

So this sentence could mean:

• My friend encrypts the backup copy
• My friend is encrypting the backup copy

If you want to make right now clearer, Arabic often adds something like الآن.

The -ي means my.

So:

• صديق = friend
• صديقي = my friend

This -ي is an attached possessive pronoun. Arabic often shows possession this way instead of using a separate word for my.

Because a noun with a possessive suffix is already definite.

So Arabic says:

• صديقي = my friend

not:

• الصديقي

The suffix itself already makes the noun specific. The same idea applies to many possessed nouns in Arabic:

• كتابي = my book
• بيتي = my house

Also, nouns with this kind of suffix do not take tanwīn.

صديقي is the subject, so grammatically it is nominative.

However, when a noun has the first-person singular possessive suffix -ي, the case ending is not shown in the usual visible way. In ordinary writing, صديقي looks the same regardless of case.

So even though it is nominative here, you do not see a clear final -u as you might expect on a fully regular noun.

Because الاحتياطية is an adjective describing النسخة, and Arabic adjectives agree with the nouns they describe.

They agree in:

• definiteness
• gender
• number
• case

So here:

• النسخة = the copy
• الاحتياطية = the backup / precautionary

Since النسخة is definite, the adjective must also be definite:

• النسخة الاحتياطية = the backup copy

No. It is a noun + adjective phrase, not an iḍāfa.

Why?

Because in an iḍāfa, the first noun normally does not take الـ. But here the first word is:

• النسخة

So this is not copy of the backup or anything like that. It is simply:

• النسخة الاحتياطية
• the backup copy

A good indefinite version is:

• نسخة احتياطية = a backup copy

Because it describes النسخة, and نسخة is a feminine noun.

Arabic adjectives must match the noun they describe, so the adjective appears in the feminine form:

• احتياطي = masculine
• احتياطية = feminine

So:

• نسخة احتياطية = a backup copy
• النسخة الاحتياطية = the backup copy

Because they are the direct object of the verb.

The thing being encrypted is the backup copy, so that whole phrase is in the accusative.

In full vocalization, you could write it as:

• النُّسْخَةَ الاحْتِيَاطِيَّةَ

The adjective takes the same case as the noun it describes, so both are accusative.

In normal Arabic writing, these short case vowels are usually omitted, which is why learners often do not see them.

Because ن is a sun letter.

With sun letters, the ل of الـ is not pronounced separately; it assimilates to the next consonant. So:

• النسخة is pronounced an-nuskhah

But in الاحتياطية, the ل is still pronounced:

• الاحتياطية = al-iḥtiyāṭiyyah

So the two words behave differently because the letters after الـ are different.

A careful fully vocalized reading would be approximately:

• يُشَفِّرُ صَدِيقِي النُّسْخَةَ الاحْتِيَاطِيَّةَ

Roughly:

• yushaffiru ṣadīqī an-nuskhata al-iḥtiyāṭiyyata

In normal pause at the end of the sentence, final short vowels are often dropped in pronunciation, so learners may hear something closer to:

• yushaffiru ṣadīqī an-nuskhah al-iḥtiyāṭiyyah