صديقي سعيد اليوم.

In a present-tense nominal sentence, Arabic usually leaves out is / am / are.

So صديقي سعيد اليوم literally looks like:

• my friend
• happy
• today

But in natural English, that becomes My friend is happy today.

If you wanted a past or future meaning, Arabic would usually use a form of كان or another verb, but in the present tense it is normally omitted.

صديقي breaks down into:

• صديق = friend
• ـي = my

So صديقي means my friend.

The ending ـي is a attached possessive pronoun. English uses a separate word (my), but Arabic often attaches it directly to the noun.

Because صديقي means my friend, it is already definite. Something possessed by me is specific.

In this kind of sentence:

• صديقي = the subject/topic
• سعيد = the predicate, describing the subject

In Arabic, when a definite noun is described in a sentence like this, the predicate is often indefinite:

• صديقي سعيد = My friend is happy

That is different from an adjective directly attached to a noun. For example:

• صديقي السعيد = my happy friend

There, السعيد would be a true adjective modifying friend, so it would match in definiteness.

A learner might notice that سعيد can be:

• an adjective meaning happy
• a male name, Saʿīd / Saeed

In this sentence, the natural reading is the adjective happy.

Why?

Because the sentence structure is:

• صديقي = subject
• سعيد = predicate
• اليوم = time expression

So it works smoothly as My friend is happy today.

Without vowels, Arabic writing can sometimes be ambiguous, but context usually makes the intended meaning clear.

اليوم means today, and here it functions as a time expression.

It tells you when your friend is happy.

So the structure is basically:

• صديقي = my friend
• سعيد = happy
• اليوم = today

Arabic often places time words like this at the end, but they can sometimes move for emphasis.

Yes. Arabic word order is more flexible than English word order.

These are all possible, with slightly different emphasis:

• صديقي سعيد اليوم = neutral
• صديقي اليوم سعيد = emphasizes today
• اليوم صديقي سعيد = starts with today for focus

The basic meaning stays the same: My friend is happy today.

Because صديقي is masculine singular.

The noun صديق means male friend / friend in the masculine form, so the predicate also appears in the masculine singular form:

• صديقي سعيد = My friend is happy

If the friend were female, you would normally say:

• صديقتي سعيدة اليوم = My female friend is happy today

So the predicate matches the gender and number of the subject.

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

In fully vocalized MSA, this sentence would be something like:

• صديقي سعيدٌ اليومَ

A few notes:

• صديقي is grammatically nominative as the subject, even though the ending is not written in an obvious way because of the ـي suffix.
• سعيدٌ is nominative because it is the predicate.
• اليومَ is often treated as an accusative adverbial expression of time.

In everyday print, all those short vowels are usually omitted, so you just see:

• صديقي سعيد اليوم

A careful MSA-style pronunciation is approximately:

• ṣadīqī saʿīdun al-yawma

If you pause naturally at the end of the sentence, the final short vowel is usually not pronounced, so it may sound more like:

• ṣadīqī saʿīdun al-yawm
• or, in less formal reading without full endings, ṣadīqī saʿīd il-yawm / al-yawm

The main useful parts for a learner are:

• صديقي = ṣadīqī
• سعيد = saʿīd
• اليوم = al-yawm