رحلة المواصلة تبدأ في الموعد المحدد.

It is basically:

• رحلة المواصلة = the subject, the connecting/onward trip
• تبدأ = the verb, begins / starts
• في الموعد المحدد = a prepositional phrase, at the specified time

So the sentence is structured like:

[subject] + [verb] + [time phrase]

Because the subject comes before the verb here, the verb shows full agreement with it.

Because the subject رحلة is a feminine singular noun.

• رحلة = trip / journey
• It is grammatically feminine
• So the verb must be third person feminine singular

That is why you get:

• يبدأ = he/it begins
• تبدأ = she/it begins

Here it refers to رحلة, which is feminine in Arabic.

رحلة المواصلة is an iḍāfa construction, often called a genitive or noun-link construction.

It literally consists of:

• رحلة = trip / journey / flight
• المواصلة = continuation / connection

Together, in context, it means something like:

• the connecting trip
• the onward journey
• the connecting flight if the context is air travel

The exact English translation depends on context, but grammatically it is a noun phrase built from two nouns.

Because in an iḍāfa construction, the first noun usually does not take الـ.

So:

• رحلة المواصلة = the connecting trip
• not الرحلة المواصلة in this structure

The definiteness comes from the second noun, المواصلة. When the second term of the iḍāfa is definite, the whole phrase becomes definite.

So even though رحلة has no الـ, the whole phrase is still definite.

Because المحدد is an adjective describing الموعد, and Arabic adjectives must agree with the noun they describe.

They agree in:

• definiteness
• gender
• number
• case

So:

• الموعد = the موعد, the appointment/time
• المحدد = the specified / set

Together:

• الموعد المحدد = the specified time / the set time

If the noun is definite, the adjective must also be definite.

Literally, في often means in, but in many time expressions it is better translated as at.

So:

• في الموعد المحدد = at the specified time

This is very normal Arabic usage. Prepositions do not always match English one-for-one.

المحدد is a passive participle from the verb حدّد meaning to specify, to determine, or to set.

So محدد means:

• specified
• determined
• set

In this sentence it works as an adjective describing الموعد.

So:

• الموعد المحدد = the specified/set time

With full vowels, the sentence can be written as:

رِحْلَةُ الْمُوَاصَلَةِ تَبْدَأُ فِي الْمَوْعِدِ الْمُحَدَّدِ

Here is why:

• رِحْلَةُ = nominative, because it is the main subject
• الْمُوَاصَلَةِ = genitive, because it is the second term of an iḍāfa
• تَبْدَأُ = present tense indicative
• الْمَوْعِدِ = genitive after the preposition في
• الْمُحَدَّدِ = genitive because it is an adjective matching الموعد

In everyday writing, these endings are usually not written, but they are still understood.

Yes. You could also say:

تبدأ رحلة المواصلة في الموعد المحدد

That is also correct.

Both word orders are possible in Arabic:

• رحلة المواصلة تبدأ...
• تبدأ رحلة المواصلة...

The version with the subject first can feel a little more like As for the connecting trip, it begins... or may simply emphasize the trip itself. The verb-first version is also very natural.

It is pronounced approximately tabdaʼu.

Important points:

• The ت gives ta-
• The بد gives -bd-
• The final أ is a hamza, a glottal stop
• So it is not smooth like tabda; it has a little break: tabdaʼu

This word comes from the root بدأ, meaning to begin.

Because Arabic verbs already include the subject information.

So تبدأ by itself already means she begins or it begins, depending on context.

Since the subject رحلة المواصلة is already stated, Arabic does not need an extra pronoun.

English often needs it begins, but Arabic can simply use the verb with the noun subject.

The present tense in Arabic can cover several meanings depending on context, including:

• simple present: begins
• near future: will begin
• scheduled future: is scheduled to begin

In a sentence like this, especially with travel or schedules, تبدأ often refers to a planned future event.

So even though the verb form is present tense, the real meaning may be something like starts at the scheduled time or will start at the specified time.