الكلمة الرئيسية بعد الجلسة الافتتاحية.

It can work as either, depending on context.

• In a program, schedule, heading, or slide title, it is very natural as a phrase.
• In context, it can also be understood as a nominal sentence meaning The keynote speech is after the opening session, with the present-tense is left unstated.

Arabic often leaves out the present-tense verb to be, and it also often uses short headline-style phrases where English would prefer a full sentence.

Because in Arabic, adjectives normally come after the noun they describe.

So:

• الكلمة الرئيسية = the main/keynote speech
• literally: the speech the-main

The same pattern appears again in:

• الجلسة الافتتاحية = the opening session

This is one of the most basic word-order differences from English.

Because Arabic adjectives must agree with the nouns they describe in definiteness as well as other features.

So if the noun is definite:

• الكلمة = the speech
• the adjective must also be definite: الرئيسية = the main

And:

• الجلسة = the session
• الافتتاحية = the opening

This is normal adjective agreement in Arabic:

• noun definite -> adjective definite
• noun indefinite -> adjective indefinite

For example:

• كلمة رئيسية = a keynote/main speech
• الكلمة الرئيسية = the keynote/main speech

No. It is noun + adjective, not iḍāfa.

Why not?

• الرئيسية is an adjective describing الكلمة
• both words are definite with الـ
• the adjective agrees with the noun

A true iḍāfa would look different, for example:

• كلمةُ الرئيسِ = the president's speech
• جلسةُ الافتتاحِ = the opening session in a different structural sense

A useful clue:

• If the second word is an adjective and matches the first in definiteness/gender/number, it is probably noun + adjective
• If the second word is another noun completing the meaning, it may be iḍāfa

Because they are describing feminine nouns.

Both of these nouns are grammatically feminine:

• كلمة
• جلسة

So their adjectives must also be feminine:

• رئيسي -> masculine

• رئيسية -> feminine

• افتتاحي -> masculine
• افتتاحية -> feminine

This is standard agreement in Arabic: adjectives match the noun in gender, as well as in number, definiteness, and usually case.

بعد means after.

It introduces the time relationship:

• بعد الجلسة الافتتاحية = after the opening session

Grammatically, the noun after بعد is in the genitive in full vocalization:

• بعدَ الجلسةِ الافتتاحيةِ

So even though the short vowels are usually not written, that is the underlying grammar.

A natural fully vocalized reading is:

الكلمةُ الرئيسيةُ بعدَ الجلسةِ الافتتاحيةِ

That shows the case endings more clearly:

• الكلمةُ
• الرئيسيةُ
• بعدَ
• الجلسةِ
• الافتتاحيةِ

However, in normal Arabic writing, these short vowels are usually omitted. In everyday speech, final case vowels are also often reduced or not clearly pronounced, especially in pause.

Here الـ is affected by the letter ر, which is a sun letter.

So:

• الرئيسية is pronounced roughly ar-raʾīsiyya
• not al-raʾīsiyya

By contrast:

• الجلسة keeps the l sound, because ج is a moon letter
• الافتتاحية also keeps the l sound

So a rough pronunciation of the whole phrase is:

al-kalima ar-raʾīsiyya baʿda al-jalsa al-iftitāḥiyya

Yes, كلمة can mean word, but in formal Arabic it can also mean:

• speech
• address
• remarks
• statement

So the correct meaning depends on context.

This is very common in formal or event-related language. For example, at conferences or ceremonies, كلمة often refers to a speech given by someone.