هذا الأثر مهم.

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

So:

• هذا الأثر مهم = This effect/trace is important

There is no separate present-tense is in the sentence. This is very normal in Arabic.

If you wanted the past, then Arabic would use a verb, for example:

• كان هذا الأثر مهمًّا = This effect/trace was important

هذا means this for a masculine singular noun.

Since الأثر is a masculine singular noun, هذا is the correct demonstrative to use.

Some related forms are:

• هذا = this (masculine singular)
• هذه = this (feminine singular)
• هذان / هاتان = these two
• هؤلاء = these

So هذا الأثر means this effect / this trace / this impact.

In standard Arabic, when a demonstrative like هذا directly points to a specific noun, that noun is often definite, so you commonly get:

• هذا الأثر = this effect

This is a very common pattern:
demonstrative + definite noun

For example:

• هذا الكتاب = this book
• هذا الرجل = this man

If you say هذا أثر مهم, that usually sounds more like:

• This is an important effect / trace

So the version with الـ after هذا is the normal way to say this + noun.

Because they are doing different jobs in the sentence.

• الأثر is the noun being identified: the effect / the trace
• مهم is the predicate adjective: important

In Arabic, the predicate in a sentence like this is often indefinite, so:

• هذا الأثر مهم = This effect is important

If you made مهم definite and said هذا الأثر المهم, that would usually mean:

• this important effect

That becomes a noun phrase, not a full sentence meaning is important.

So:

• هذا الأثر مهم = full sentence
• هذا الأثر المهم = noun phrase

This is a nominal sentence (جملة اسمية), meaning a sentence that begins with a noun-like element rather than a verb.

A simple learner-friendly breakdown is:

• هذا الأثر = the topic / subject-like part
• مهم = the comment / predicate

A more traditional grammar explanation is:

• هذا = demonstrative pronoun
• الأثر = often analyzed as an appositional noun explaining هذا
• مهم = the predicate (خبر)

But for learning purposes, it is enough to understand the whole sentence as:

• This effect is important

It is pronounced approximately al-athar.

A few useful notes:

• ث sounds like the th in think
• The ال here is pronounced clearly because ث is a moon letter, not a sun letter
• So you say al-athar, not ath-thar

A rough pronunciation of the whole sentence is:

• hādhā al-athar muhimm

Arabic is usually written without short vowels in normal texts. So you see:

• مهم

But the fully vowelled form here is:

• مُهِمٌّ

And the full sentence with vowels is:

• هَذَا الْأَثَرُ مُهِمٌّ

The final -u / -un endings show that both الأثر and مهم are in the nominative in this sentence.

In everyday reading and speaking, especially in pauses, these endings are often not pronounced.

In full formal Arabic, the sentence is:

• هَذَا الْأَثَرُ مُهِمٌّ

Here is the grammatical case information:

• هذا: indeclinable in this form, so its ending does not change
• الأثرُ: nominative
• مهمٌّ: nominative

Why nominative? Because in a nominal sentence:

• the subject/topic part is nominative
• the predicate is also nominative

In normal spoken reading, many learners will simply say:

• هذا الأثر مهم

without fully pronouncing the endings.

Not always. أثر has a range of meanings depending on context. It can mean:

• effect
• impact
• trace
• mark
• remnant
• influence

So هذا الأثر مهم could mean different things depending on context, such as:

• This effect is important
• This trace is important
• This impact is important

If the meaning has already been provided in your lesson, that meaning is probably the intended one for this sentence.

Some changes are possible, but they may change the meaning or sound less natural.

The standard sentence is:

• هذا الأثر مهم = This effect is important

You can also say:

• الأثر مهم = The effect is important
• هذا مهم = This is important

But if you remove الـ and say:

• هذا أثر مهم

that usually means:

• This is an important effect

So small changes in word order or definiteness can change the structure and meaning.

Arabic nouns have grammatical gender, and أثر is masculine.

That is why the sentence uses:

• هذا and not هذه

If the noun were feminine, you would use هذه instead. For example:

• هذه الفكرة مهمة = This idea is important

So when learning a new Arabic noun, it is very helpful to learn its gender at the same time.