صاحبي اسف.

It is the possessive suffix meaning my.

So:

• صاحب = friend / companion
• صاحبي = my friend

In Levantine, attaching -ي directly to a noun is a very common way to say possession.

Yes. صاحبي is very natural in colloquial Levantine and often feels like my friend, my buddy, or my mate.

A more formal or Standard Arabic way would be صديقي. So:

• صاحبي = more colloquial, everyday
• صديقي = more formal / MSA

Because Arabic usually drops the present-tense verb to be.

So صاحبي اسف literally looks like:

• my friend sorry

but it means:

• my friend is sorry

This is totally normal in Arabic nominal sentences.
If you wanted past tense, then a verb would appear, for example with كان (was).

A rough pronunciation is:

SAAH-bee AA-sif

A slightly more accurate transliteration is:

ṣāḥbi āsif

A few sound notes:

• ص is a heavier, emphatic s
• ح is a breathy throaty sound that English does not really have
• صاحبي is often said like ṣāḥbi
• آسف is āsif

If you cannot make ص and ح perfectly yet, that is normal for beginners.

In careful spelling, yes: آسف is the standard spelling.

In casual texting or informal writing, people often simplify spellings and write اسف instead. Native speakers will still understand it easily.

So:

• آسف = standard spelling
• اسف = very common informal spelling

Yes. It agrees with the person being described.

• آسف = masculine
• آسفة = feminine

So in your sentence, آسف tells you the friend is male.

If the friend were female, you would say:

صاحبتي آسفة

One extra note: in some contexts صاحبتي can also mean my girlfriend, so context matters.

It is very understandable across the Arabic-speaking world, but it feels colloquial because of صاحبي.

• آسف exists in both Standard Arabic and many dialects
• صاحبي feels more spoken and regional/colloquial

A more Standard Arabic version would be:

صديقي آسف

So the sentence is very natural for spoken Arabic, including Levantine.

Because صاحبي is already definite.

Once you add -ي (my), the noun becomes specific automatically:

• صاحب = a friend / friend
• صاحبي = my friend

So you do not add الـ to it.

Yes, depending on context.

In Levantine, صاحبي can mean:

• my friend
• my buddy
• sometimes my boyfriend / partner

Usually the surrounding context makes the meaning clear. In a simple learning sentence like this, it is usually understood as my friend.

Yes. The structure is:

• صاحبي = subject
• آسف = predicate adjective

So the pattern is basically:

[noun] + [adjective/description]

That is a very common Arabic sentence pattern.
It is normal for the describing word to come after the noun here.