التحديد مهم في الخطة.

It is a nominal sentence in Arabic, meaning a sentence built without a main verb in the present tense.

Its basic structure is:

• التحديد = the topic / subject (mubtadaʾ)
• مهم = the comment / predicate (khabar)
• في الخطة = a prepositional phrase giving extra information

So the sentence is literally structured like:

التحديد + important + in the plan

In Arabic, the verb to be is usually not written in the present tense.

So:

• التحديد مهم = Specification / precision is important
• literally: the specification important

This is completely normal in Arabic.
If you wanted the past tense, then Arabic would use a verb such as كان:

• كان التحديد مهمًّا = Specification was important

Because التحديد is the subject/topic, and مهم is the predicate.

In Arabic, it is very common for:

• the subject to be definite
• the predicate to be indefinite

So:

• التحديد = the specification / the defining / being specific
• مهم = important

You would usually not say التحديد المهم here, because that would mean the important specification, which is a noun phrase, not a full sentence.

Compare:

• التحديد مهم = Specification is important
• التحديد المهم = the important specification

التحديد is a verbal noun (maṣdar) from Form II حدّد.

It can carry meanings such as:

• definition
• specification
• precision
• determining
• being specific

The exact English translation depends on context, but grammatically it is functioning here as a noun, not as a verb.

Because it matches التحديد, which is treated as a masculine singular noun.

In Arabic, adjectives and predicate adjectives usually agree in:

• gender
• number
• sometimes definiteness, depending on the structure

Here, مهم is singular masculine because التحديد is singular masculine.

If the subject were feminine, you would expect مهمة instead.

في الخطة is a prepositional phrase meaning in the plan.

• في = in
• الخطة = the plan

This phrase adds context to the main statement.
The core sentence is:

• التحديد مهم = Specification is important

Then في الخطة tells you where / in what context it is important.

Because الخطة comes after the preposition في, and nouns after prepositions are normally in the genitive case in Arabic.

So the fully vocalized sentence is:

التحديدُ مهمٌّ في الخطةِ

Case endings:

• التحديدُ = nominative, because it is the subject
• مهمٌّ = nominative, because it is the predicate
• الخطةِ = genitive, because it follows في

In normal unvowelled writing, these endings are usually not shown.

The l of الـ is not pronounced here, because ت is a sun letter.

So التحديد is pronounced approximately:

at-taḥdīd

not:

al-taḥdīd

This is a common pronunciation rule in Arabic:

• with sun letters, the l assimilates
• with moon letters, the l is pronounced

Also notice the shadda-like doubling effect on the t sound in pronunciation.

Yes, Arabic allows some flexibility, but the given order is the most straightforward and neutral.

Normal order here:

• التحديد مهم في الخطة

You may also see reordering for emphasis, especially in formal or literary Arabic, such as fronting the prepositional phrase:

• في الخطةِ التحديدُ مهمٌّ

This can sound more like:

• In the plan, specification is important

But for learners, the original order is the best one to start with.

Yes, it could be either, depending on meaning.

In your sentence:

• في الخطة = in the plan

This suggests a specific plan already known from context.

If you wanted in a plan, you would say:

• في خطةٍ

So the choice between الخطة and خطة depends on whether the noun is:

• definite = a particular plan
• indefinite = any plan / a plan

The fully vocalized form is:

التحديدُ مهمٌّ في الخطةِ

Breakdown:

• التحديدُ — -u ending, nominative
• مهمٌّ — -un ending, nominative and indefinite
• في — preposition
• الخطةِ — -i ending, genitive after في

In everyday print, you will usually just see:

التحديد مهم في الخطة

without the short vowels.