هذا مصدر مهم.

A common pronunciation is:

hādhā maṣdar muhimm

If you include the full grammatical endings, it is:

hādhā maṣdarun muhimmun

A few notes:

• هذا = hādhā
• مصدر = maṣdar, with a strong ṣ sound
• مهم = muhimm, and the last m is doubled because of the shadda on م

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

So:

• هذا مصدر مهم literally looks like this source important
• but it means This is an important source

This is called a nominal sentence. In the present tense, Arabic normally leaves out is / am / are.

If you wanted was, Arabic would use a verb, for example كان.

هذا means this.

In this sentence, it functions like this is in English:

• هذا مصدر مهم = This is an important source

So هذا is pointing to something nearby, masculine singular.

If you were pointing to a feminine singular noun, you would usually use هذه instead.

This is an important distinction.

• هذا مصدر مهم = This is an important source
• هذا المصدر مهم = This source is important

In your sentence, مصدر is indefinite because it is part of the predicate: you are identifying what this is.

So the pattern is:

• هذا + indefinite noun/adjective phrase
• meaning This is a/an ...

But if you want this source, the noun itself is definite:

• هذا المصدر

Because Arabic adjectives normally come after the noun they describe.

So:

• مصدر مهم = an important source

This is the normal order in Arabic:

• noun first
• adjective second

Unlike English, where the adjective usually comes before the noun.

Yes. Arabic adjectives must agree with the noun they describe.

Here, مصدر is:

• masculine
• singular
• indefinite

So مهم is also:

• masculine
• singular
• indefinite

That is why مهم is the correct form here.

If the noun were feminine, you would usually use مهمة instead.

With full case endings, the sentence is:

هذا مصدرٌ مهمٌّ

Here is why:

• هذا does not change here
• مصدرٌ is in the nominative case
• مهمٌّ is also nominative because it describes مصدر, and adjectives match the nouns they modify

In normal Arabic writing, these short vowel endings are usually not written, so you often just see:

هذا مصدر مهم

Yes. مصدر has more than one common meaning.

In general usage, it often means:

• source

In Arabic grammar, it can also mean:

• verbal noun
• something similar to an infinitive in some contexts

So the exact meaning depends on context. In this sentence, it most naturally means source.

Because it does.

The word مهم is actually pronounced muhimm, not just muhim. The doubled m comes from the shadda on the last م:

• مُهِمّ

A shadda means the consonant is doubled or held a little longer.

So a careful pronunciation is:

• mu-himm

It means This is an important source.

That is because the sentence is complete:

• هذا = this
• مصدر مهم = an important source

So it is a full statement.

If you wanted just the noun phrase this important source, you would normally say:

هذا المصدر المهم

That phrase is not a full sentence by itself; it is just a noun phrase.