بصراحة، هذا مصدر مهم.

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

So هذا مصدر مهم literally looks like this important source, but it means this is an important source.

This is called a nominal sentence in Arabic:

• هذا = the subject
• مصدر مهم = the predicate

If you wanted a past-tense version, Arabic would use a verb, for example:

• كان هذا مصدرًا مهمًّا = This was an important source

Because مصدر is a masculine singular noun in Arabic.

The demonstrative has to match the gender and number of the noun:

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

So:

• هذا مصدر = this is a source
• هذه مجلة = this is a magazine

بصراحة literally means something like with frankness or with honesty.

It is made up of:

• بـ = with / in
• صراحة = frankness / honesty / openness

As a whole, it is commonly used like:

• Honestly
• Frankly
• To be honest

So in this sentence, بصراحة sets the tone: Honestly, this is an important source.

Because in 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

Also, the adjective agrees with the noun in:

• gender
• number
• definiteness
• case

Here both are:

• masculine
• singular
• indefinite

So مصدر and مهم match properly.

They are indefinite because neither one has الـ.

• مصدر = a source
• مهم = important

So مصدر مهم means an important source, not the important source.

If you made them definite, the meaning would change:

• المصدر المهم = the important source

In this sentence, the speaker is identifying this as belonging to the category an important source, so the indefinite form is natural.

In fully vocalized Modern Standard Arabic, the sentence would be:

بِصَراحَةٍ، هذا مَصْدَرٌ مُهِمٌّ.

Here is why:

• بِصراحةٍ: after the preposition بـ, the noun is in the genitive
• هذا: demonstratives are basically fixed in form
• مصدرٌ: nominative, because it is the predicate of a nominal sentence
• مهمٌّ: nominative too, because it is an adjective matching مصدر

In normal writing, these short vowel endings are usually omitted.

It is pronounced roughly muhimm.

A few things to notice:

• the م at the end is doubled because of the shadda: مهمّ
• the stress is usually on the second syllable: mu-HIMM

So:

• مهم = muhimm

Learners often miss the doubled final consonant, but it matters in careful pronunciation.

A rough pronunciation guide is:

• بصراحة = bi-ṣa-rā-ḥa
• هذا = hā-dhā

A few sound notes:

• ص is an emphatic s, heavier than ordinary س
• ح is a strong breathy ḥ sound, not the English h
• ذ in هذا sounds like th in this

So the whole sentence is roughly:

bi-ṣarāḥa, hādhā maṣdar muhimm

In formal full pronunciation with endings, it would be closer to:

bi-ṣarāḥatin, hādhā maṣdarun muhimmun

Yes. Both can mean honestly / frankly.

• بصراحة is very common and natural
• صراحةً is also correct and common in formal Arabic

So both of these work:

• بصراحة، هذا مصدر مهم.
• صراحةً، هذا مصدر مهم.

The first one may feel slightly more conversational in many contexts, but both are perfectly understandable in MSA.

You would usually use ليس:

بصراحة، هذا ليس مصدرًا مهمًّا.
= Honestly, this is not an important source.

Notice that after ليس:

• مصدرًا becomes accusative
• مهمًّا also becomes accusative to match it

So this is a useful comparison:

• هذا مصدرٌ مهمٌّ = This is an important source
• هذا ليس مصدرًا مهمًّا = This is not an important source

It is a complete sentence.

Even though there is no written verb for is, Arabic allows full present-tense sentences without it.

So:

• بصراحة = an introductory adverbial expression
• هذا مصدر مهم = a full nominal sentence

Together they form a complete statement: Honestly, this is an important source.