هذا السؤال سهل، لكن السؤال الثاني صعب.

In Arabic, the present tense of to be is usually not written or spoken in simple sentences like this.

So:

• هذا السؤال سهل literally looks like this question easy
• but it means This question is easy

This is called a nominal sentence in Arabic.
If you want was or will be, then Arabic normally uses a verb such as كان.

هذا means this and is the masculine singular demonstrative.

It comes before the noun:

• هذا السؤال = this question

This is the normal Modern Standard Arabic pattern for this/that + noun.

Also, السؤال is a masculine singular noun, so هذا is the correct form. If the noun were feminine, Arabic would usually use هذه instead.

الـ is the definite article, like the in English.

Here it appears because both nouns are definite:

• هذا السؤال = this question
• السؤال الثاني = the second question

After a demonstrative like هذا, Arabic normally uses a definite noun:

• هذا السؤال = this question
• not usually هذا سؤال if you specifically mean this question

By contrast, هذا سؤال more naturally means this is a question.

Because here they are functioning as predicates: they tell you something about the subject.

• هذا السؤال سهل = This question is easy
• السؤال الثاني صعب = The second question is difficult

In this kind of sentence, the describing word is often indefinite, so it usually does not take الـ.

So:

• السؤال سهل = The question is easy
• not السؤال السهل here, because السؤال السهل would mean the easy question, which is a noun phrase, not a full sentence.

They match السؤال, which is masculine singular.

In Arabic, adjectives and similar descriptive words usually agree with the noun in:

• gender
• number
• definiteness (in many adjective phrases)

Since السؤال is masculine singular, the forms are:

• سهل
• صعب

If the noun were feminine, you would typically see feminine forms, such as سهلة or صعبة.

الثاني means the second. It is an ordinal number, and in Arabic ordinals behave much like adjectives.

That means they usually come after the noun they describe:

• السؤال الثاني = the second question

It also agrees with the noun:

• السؤال is masculine singular and definite
• so Arabic uses الثاني, also masculine singular and definite

This is the normal order in Arabic:

• noun + ordinal/adjective

not usually the English order second question.

Repeating السؤال makes the sentence clear, balanced, and natural in Modern Standard Arabic.

So Arabic says:

• هذا السؤال سهل، لكن السؤال الثاني صعب

This is very normal formal style.

Could Arabic sometimes leave the noun out if the meaning is obvious? In some contexts, yes. But for learners, formal writing, and clear contrast, repeating the noun is the safest and most natural choice.

لكن means but and introduces a contrast between the two parts:

• This question is easy
• but the second question is difficult

So it links the two clauses in the same way but does in English.

A useful extra note: in fully vocalized grammar discussions, learners may also meet لكنَّ. In ordinary unvocalized Arabic writing, both are often written simply as لكن, which is one reason this word can be confusing at first.

A natural pronunciation is:

hādhā as-suʾāl sahl, lākin as-suʾāl ath-thānī ṣaʿb

Two useful pronunciation points:

• In السؤال, the س is a sun letter, so ال is pronounced as-, not al-
• In الثاني, the ث is also a sun letter, so ال is pronounced ath-

So:

• السؤال sounds like as-suʾāl
• الثاني sounds like ath-thānī

Also, سؤال contains a glottal stop in the middle: su-ʾāl.

It is a nominal sentence because it does not begin with a verb.

Each half is essentially a small nominal sentence:

• هذا السؤال سهل
• السؤال الثاني صعب

In Arabic grammar, these are built from a subject/topic and a predicate/comment rather than from a verb.

That is why the sentence can express is without using an actual present-tense verb.