التحقيق صعب.

Because Arabic often leaves out the verb to be in the present tense.

So:

• التحقيق صعب = The investigation is difficult

This is a very common Arabic sentence pattern called a nominal sentence. It has:

• التحقيق = the subject/topic
• صعب = the comment/predicate

In the present tense, Arabic normally does not say a separate word for is.

But in other tenses, a verb can appear:

• كان التحقيق صعبًا = The investigation was difficult
• سيكون التحقيق صعبًا = The investigation will be difficult

It is a nominal sentence in Arabic, meaning a sentence built around a noun rather than beginning with a verb.

More specifically:

• التحقيق is the مبتدأ (mubtadaʼ) — the subject/topic
• صعب is the خبر (khabar) — the predicate/comment

So the structure is:

• [subject] + [predicate]
• التحقيق + صعب

This is one of the most basic and important sentence patterns in Modern Standard Arabic.

Because in this sentence, التحقيق is the subject and صعب is the predicate adjective.

• التحقيق = the investigation
• صعب = difficult

In Arabic nominal sentences, the subject is often definite, while the predicate is often indefinite.

So:

• التحقيق صعب = The investigation is difficult

If you made both words definite:

• التحقيق الصعب

that would no longer mean The investigation is difficult. It would mean:

• the difficult investigation

So this is an important difference:

• التحقيق صعب = The investigation is difficult
• التحقيق الصعب = the difficult investigation

It is an adjective in meaning, but grammatically here it functions as the predicate of the sentence.

That means it is not simply attached to the noun as part of one noun phrase. Instead, it is telling you something about the subject.

Compare:

• التحقيق صعب = The investigation is difficult

  • whole sentence
  • صعب is the predicate

• التحقيق الصعب = the difficult investigation

  • noun phrase, not a complete sentence by itself
  • الصعب is an attributive adjective modifying التحقيق

So the same adjective can appear in two different roles.

A careful MSA pronunciation is approximately:

• at-taḥqīqu ṣaʿbun with full case endings
• or more commonly in pause: at-taḥqīq ṣaʿb

A few pronunciation notes:

• ال before ت is assimilated, so التحقيق is pronounced at-taḥqīq, not al-taḥqīq
• ح is a voiceless deep h sound, stronger than English h
• ق is q, pronounced farther back than k
• ص is an emphatic s
• ع in صعب is the letter ʿayn, a sound English does not have

If you are reading without case endings, which is very common, you will often hear:

• at-taḥqīq ṣaʿb

In fully vocalized formal Arabic, it is:

• التَّحْقِيقُ صَعْبٌ

The endings are:

• التَّحْقِيقُ = -u ending, because it is the subject of the nominal sentence
• صَعْبٌ = -un ending, because it is an indefinite predicate

In normal modern writing, these short vowels are usually omitted, so you usually see:

• التحقيق صعب

But it is useful to know the underlying full form.

Because most everyday Arabic writing leaves out short vowels and tanwīn.

The fully vocalized form is:

• صَعْبٌ

But normally it is written simply as:

• صعب

This does not mean the tanwīn is absent grammatically. It just means it is not written.

So when learning grammar, you should understand that the full formal ending is there in principle, even if ordinary text does not show it.

Because it agrees with التحقيق, which is grammatically masculine singular.

In Arabic, adjectives and predicate adjectives usually agree with the noun in:

• gender
• number

Since التحقيق is masculine singular, the predicate is also masculine singular:

• التحقيق صعب

If the subject were feminine, the adjective would change, for example:

• المهمة صعبة = The task is difficult

So صعب is the correct form here because التحقيق is masculine singular.

التحقيق comes from the root ح-ق-ق, which is connected with meanings like:

• truth
• verification
• establishing something as true or real

In different contexts, تحقيق can mean things like:

• investigation
• inquiry
• verification
• realization/fulfillment

So this word has a broad semantic range, depending on context.

For a learner, it is useful to notice the connection between:

• حق = truth/right
• حقيقة = truth, reality
• تحقيق = investigation, verification, realization

This can help you remember the vocabulary more deeply.

Not if you want to keep exactly the same meaning and structure.

• التحقيق صعب means The investigation is difficult
• صعب التحقيق usually means something more like hard to achieve/verify/investigate, depending on context, and it is not the same sentence structure

If you simply want a standard sentence meaning The investigation is difficult, use:

• التحقيق صعب

Arabic can vary word order for stylistic reasons, but for a basic learner sentence, this is the normal and safest form.

A common way is to use ليس:

• التحقيق ليس صعبًا = The investigation is not difficult

Notice that صعب becomes:

• صعبًا

because after ليس, the predicate is in the accusative in full formal grammar.

In everyday unvocalized writing, you will usually just see:

• التحقيق ليس صعبا
• or even التحقيق ليس صعب

depending on the level of formality and spelling habits

But the standard fully grammatical form is:

• التحقيق ليس صعبًا

You can add a form of كان:

• كان التحقيق صعبًا = The investigation was difficult
• سيكون التحقيق صعبًا = The investigation will be difficult

In these sentences:

• التحقيق remains the subject
• صعبًا is the predicate in the accusative in full formal grammar

So this is a very useful pattern to learn:

• present: التحقيق صعب
• past: كان التحقيق صعبًا
• future: سيكون التحقيق صعبًا

The difference is definiteness.

• التحقيق صعب = The investigation is difficult
• تحقيق صعب = a difficult investigation or difficult verification/investigation, depending on context

So:

• with ال on التحقيق, you have a definite subject and a full sentence
• without ال, تحقيق صعب is usually just a noun phrase, not a complete sentence in the same way

This is an important distinction in Arabic, because the presence or absence of ال can completely change the structure and meaning.