الكذب سيئ.

In present-tense Arabic nominal sentences, the verb to be is usually not written or spoken.

So:

• الكذب سيئ literally looks like lying bad
• but it means lying is bad

If you want a past meaning, you would use a verb such as كان:

• كان الكذبُ سيئًا = Lying was bad

It is a nominal sentence in Arabic, called جملة اسمية.

It has two main parts:

• الكذب = the topic/subject (مبتدأ)
• سيئ = the predicate/comment (خبر)

So the structure is:

• [subject] + [predicate]
• الكذبُ سيئٌ

Because Arabic often uses الـ with abstract or general ideas.

Here, الكذب does not have to mean only the lie in a specific situation. It can mean lying or falsehood in general.

This is called a generic definite use of الـ.

A similar pattern is:

• الصدقُ فضيلةٌ = Honesty is a virtue
• literally: the honesty is a virtue

So الـ here helps express the general concept.

Because سيئ here is not an adjective directly attached to the noun. It is the predicate of the sentence.

That is an important difference:

• الكذبُ سيئٌ = Lying is bad
  
  • complete sentence
• الكذبُ السيئُ = the bad lying / the bad falsehood
  
  • noun + adjective phrase, not a full sentence by itself

When an adjective directly modifies a noun in Arabic, it usually matches it in definiteness. But here سيئ is not a modifier; it is what is being said about the subject.

In Arabic, adjectives normally come after the noun they describe.

But in this sentence, سيئ is not simply an adjective inside a noun phrase. It is the predicate of the whole sentence.

So Arabic is not saying:

• bad lying

It is saying:

• lying [is] bad

That is why the structure feels different from English.

In fully vowelled MSA, it is commonly read as:

الكَذِبُ سَيِّئٌ
al-kadhibu sayyiʾun

A rough pronunciation guide:

• al-kadhibu = al-KA-dhi-bu
• sayyiʾun = say-YI-un, with a clear glottal stop before the ending

A few pronunciation notes:

• the يّ has a shadda, so the y sound is doubled
• the final ء is a hamza, a small catch in the throat
• in pause, the final case ending is often dropped, so you may hear sayyiʾ

In full grammatical MSA, both words are nominative:

• الكذبُ = -u
• سيئٌ = -un

Why?

• الكذبُ is the mubtadaʾ (subject/topic), so it is nominative
• سيئٌ is the khabar (predicate), so it is also nominative

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

• الكذب سيئ

No. It is a noun.

More specifically, it is a verbal noun or masdar related to the verb كَذَبَ = to lie.

So الكذب behaves like a noun, even though English may translate it with something that looks verbal, like lying.

That is very common in Arabic.

Because الكذب is grammatically treated as a masculine singular noun.

In Arabic, predicates agree with the grammatical properties of the subject, so the natural form is the masculine singular سيئ.

Even though lying in English feels like an action, Arabic handles الكذب as a regular noun.

You usually should keep the basic order:

• الكذبُ سيئٌ

That is the normal, neutral way to say it.

A reversed order such as:

• سيئٌ الكذبُ

is possible in more literary or emphatic contexts, but it is not the default form learners should start with.

You can use ليس:

• الكذبُ ليس سيئًا = Lying is not bad

Notice the grammar:

• الكذبُ stays nominative
• سيئًا becomes accusative after ليس

So the ending of the predicate changes in fully vowelled MSA.

Yes. The ك in الكذب is a moon letter, not a sun letter, so the ل of الـ is pronounced clearly.

So you say:

• al-kadhib
• not something like ak-kadhib

That is why this word begins with a clear al- sound.