صديقي يشترك اليوم.

The ending ـي is the attached pronoun meaning my. So:

• صديق = friend
• صديقي = my friend

This ending is pronounced -ī, so the word is roughly ṣadīqī or sadeeqi.

Because a noun with a possessive suffix is already definite. In Arabic, my friend is automatically definite, so you do not say الصديقي.

So:

• صديق = a friend / friend
• صديقي = my friend

Possession itself makes the noun definite.

A full MSA pronunciation is:

ṣadīqī yashtariku al-yawma

A simple English-style transliteration would be:

sadeeqi yashtariku al-yawma

In normal speech, the last short vowel is often dropped in pause, so you may hear something closer to al-yawm at the end.

Because the verb يشترك already tells you the subject is he. Arabic verbs usually include person, number, and gender information.

So يشترك means he ... in the present/imperfect. Arabic often leaves out separate subject pronouns unless there is a reason to emphasize them.

يشترك is:

• a present/imperfect verb
• 3rd person masculine singular
• from the root ش ر ك
• in Form VIII: اشترك / يشترك

That means the verb pattern itself adds part of the meaning, and the ت in يشترك is part of that pattern, not a separate word.

This is a normal Arabic structure. When the sentence starts with صديقي, it is a nominal sentence with:

• صديقي as the topic/subject
• يشترك اليوم as the predicate/comment about it

Arabic could also say يشترك صديقي اليوم. Both are possible. Starting with صديقي puts a little more focus on my friend.

اليوم is a time expression, functioning like today. In grammar terms, it is an adverbial expression of time.

It keeps الـ because this word is literally the day, and Arabic uses it idiomatically to mean today.

In fully vowelled formal Arabic, it is often written الْيَوْمَ.

In normal Arabic writing, short vowels are usually not written. So learners often see:

صديقي يشترك اليوم

If fully vowelled, it would be:

صَدِيقِي يَشْتَرِكُ الْيَوْمَ

That shows the pronunciation more clearly, but ordinary texts usually omit those marks.

Yes. Depending on context, يشترك can mean things like:

• participates
• joins
• subscribes
• sometimes shares in something

Arabic often relies on context to make the intended meaning clear. So the exact English translation of يشترك can change from one sentence to another.

It would change to the feminine form:

• صديقي يشترك اليوم = with a masculine subject
• صديقتي تشترك اليوم = with a feminine subject

So the beginning of the verb changes from يـ to تـ for she in the present/imperfect.