هذا موضوع مهم.

In the present tense, Arabic usually does not use a separate word for is / am / are.

So:

• هذا موضوع مهم = This is an important topic

Arabic simply puts the parts next to each other. This kind of sentence is called a nominal sentence.

If you wanted a past or future meaning, Arabic would use other words, but for a simple present statement like this, no verb is needed.

Here is the breakdown:

• هذا = this
• موضوع = topic / subject
• مهم = important

So the sentence literally looks like:

• this topic important

But the natural English meaning is:

• This is an important topic

Because موضوع is a masculine singular noun in Arabic.

Arabic demonstratives have to match gender:

• هذا = this for masculine singular
• هذه = this for feminine singular

So:

• هذا موضوع مهم = This is an important topic
• If the noun were feminine, you would use هذه

For example:

• هذه فكرة مهمة = This is an important idea

Here مهم is describing موضوع.

So:

• موضوع مهم = an important topic

That whole phrase acts as the predicate after هذا.

A helpful way to see it is:

• هذا = this
• موضوع مهم = an important topic

So the sentence structure is really:

• This = an important topic

Not:

• This = topic and then separately important

Because they are indefinite here.

• موضوع = a topic
• الموضوع = the topic
• مهم = important (indefinite form)
• المهم = the important / the important one depending on context

In هذا موضوع مهم, the meaning is:

• This is an important topic

not:

• This is the important topic

So the noun phrase is indefinite: موضوع مهم = an important topic.

This is a very common and important distinction.

1. هذا موضوع مهم

This means:

• This is an important topic

Here, موضوع is indefinite. The sentence is identifying what this is.

2. هذا الموضوع مهم

This means:

• This topic is important

Here, هذا الموضوع is a single noun phrase meaning this topic, because الموضوع is definite.

So the difference is:

• هذا موضوع مهم = This is an important topic
• هذا الموضوع مهم = This topic is important

That little ال changes the structure a lot.

Yes. Arabic adjectives normally agree with the noun they describe in:

• gender
• number
• definiteness
• case

Here:

• موضوع is masculine
• singular
• indefinite

So مهم must also be:

• masculine
• singular
• indefinite

That is why you get:

• موضوع مهم

If the noun were feminine, you would say:

• فكرة مهمة = an important idea

Notice how مهم changes to مهمة to match the feminine noun.

A common pronunciation is:

• hādhā mawḍūʿ muhimm

If you read it with full case endings in careful MSA, it is:

• hādhā mawḍūʿun muhimmun

A few notes:

• هذا = hādhā
• موضوع = mawḍūʿ
• مهم = muhimm

In normal speech, final short vowels are often not pronounced, especially at the end of a phrase or sentence.

In fully vocalized Arabic, the sentence is:

• هٰذا مَوْضوعٌ مُهِمٌّ

The endings -un on موضوعٌ and مهمٌّ show that they are in the nominative.

Why you usually do not see them:

• Most everyday Arabic writing leaves out short vowels.
• Readers are expected to know them from grammar and context.

So in ordinary writing you usually see:

• هذا موضوع مهم

but in careful grammatical form it is:

• هٰذا مَوْضوعٌ مُهِمٌّ

No, هذا is basically indeclinable in normal usage.

That means it stays هذا regardless of its role in the sentence.

This can feel strange to English speakers learning Arabic grammar, because many Arabic words do change endings for case. But demonstratives like هذا are much more fixed in form.

So even though موضوع and مهم can show case endings in full vocalization, هذا itself stays the same.