صديقي قادر.

It is صديق + ي.

• صديق = friend
• ـي = my

So صديقي literally means my friend. The ـي here is an attached possessive pronoun.

Because صديقي is already definite. In Arabic, when a noun has a possessive ending like ـي (my), it does not also take الـ.

So Arabic says:

• صديقي = my friend

not

• الصديقي

In the present tense, Arabic usually does not use a word for is/am/are in simple statements like this. This is a very common feature of Arabic nominal sentences.

So:

• صديقي قادر = My friend is capable

with no separate word for is.

If you wanted a past meaning, Arabic would normally use a verb such as كان.

قادر is an active participle and is often used like an adjective. Here it means capable / able.

In this sentence, it functions as the predicate of the sentence, describing صديقي.

Because here قادر is the predicate of the sentence, not an adjective directly attached to the noun.

In Arabic, with a sentence like this:

• صديقي قادر

the subject is definite (my friend), and the predicate is commonly indefinite (capable).

If you said صديقي القادر, that would usually be understood more like my capable friend or my friend who is the capable one, not the simple statement my friend is capable.

The key clue is definiteness.

• صديقي is definite because of ـي (my)
• قادر is indefinite because it has no الـ

In Arabic, an adjective that directly describes a definite noun usually must also be definite. So:

• صديقي قادر = My friend is capable
• صديقي القادر = my capable friend

That difference is very important.

Yes. It agrees in gender and number.

Here both are masculine singular, so قادر is masculine singular too.

For example:

• صديقتي قادرة = My female friend is capable

So the form changes if the noun changes.

It is in the nominative case because it is the predicate of a nominal sentence.

In full vocalization, it would be:

• قادرٌ

But in normal Arabic writing, short vowels are usually not written, so you often just see:

• قادر

And in pause, speakers normally pronounce it simply as qādir, without the final -un.

A common pronunciation is:

• ṣadīqī qādir

If fully vocalized in careful grammar, it is:

• ṣadīqī qādirun

A few pronunciation notes:

• ṣ in ṣadīqī is an emphatic s
• q in qādir is ق, pronounced farther back than English k
• ī and ā are long vowels

It is a nominal sentence because it begins with a noun, not a verb.

Its basic structure is:

• صديقي = the subject
• قادر = the predicate

This type of sentence is extremely common in Arabic.