المستقبل مهم.

A natural pronunciation is al-mustaqbal muhimm.

In very careful, fully inflected MSA, it is al-mustaqbalu muhimmun.

A couple of useful details:

• In المستقبل, the ل of الـ is pronounced, because م is a moon letter, not a sun letter.
• مهم is really مُهِمّ when fully vocalized, with a shadda on the last م, so it sounds like muhimm, not muhim.

Because Arabic usually leaves out the verb to be in the present tense.

So:

• المستقبل مهم = The future is important

Literally, it looks more like the future important, but that is completely normal in Arabic.

If you wanted the past or future, then Arabic would use a verb:

• كان المستقبل مهمًّا = The future was important
• سيكون المستقبل مهمًّا = The future will be important

Because المستقبل is the subject/topic, while مهم is the predicate.

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

Compare:

• المستقبل مهم = The future is important
• المستقبل المهم = the important future

That second one is just a noun phrase, not a complete sentence.

It is a nominal sentence: a sentence that begins with a noun, not a verb.

Its two main parts are:

• المستقبل = the مبتدأ (mubtadaʾ, the subject/topic)
• مهم = the خبر (khabar, the predicate)

So the structure is:

• [subject] + [predicate]

Very common in Arabic.

Yes. مهم is an adjective meaning important.

Here, it is being used predicatively, meaning it functions as the predicate of the sentence:

• المستقبل مهم = The future is important

When an adjective comes directly after a noun and matches it in definiteness, it is attributive instead:

• المستقبل المهم = the important future

So the same adjective can be used in two different ways.

Because it matches المستقبل, which is a masculine singular noun.

Arabic adjectives normally agree with the noun they describe or refer to in:

• gender
• number
• definiteness (when used attributively)

Here the noun is masculine singular, so the predicate adjective is also masculine singular:

• المستقبل مهم

If the noun were feminine, you would usually use the feminine form:

• الحياة مهمة = Life is important

In fully vocalized MSA, the sentence is:

• المستقبلُ مهمٌّ

Both words are in the nominative:

• المستقبلُ because it is the مبتدأ
• مهمٌّ because it is the خبر

In normal writing, these short vowel endings are usually omitted, so you just see:

• المستقبل مهم

Also, at the end of a sentence, speakers often pause, so مهمٌّ may be pronounced simply as muhimm.

The basic, neutral order is:

• المستقبل مهم

That is the best pattern for a learner to use first.

Arabic can sometimes put the predicate first for emphasis in formal or literary styles, but that is not the usual beginner pattern. So for everyday MSA learning, stick with:

• المستقبل مهم

A standard way is:

• المستقبل ليس مهمًّا = The future is not important

Notice that after ليس, the predicate becomes مهمًّا.

Another common way is:

• المستقبل غير مهم = The future is unimportant / not important

Both are useful, but ليس is especially important to learn because it is a basic negation pattern for nominal sentences.

If you remove الـ, you get مستقبل, which is usually a future or future in some context, not the future.

So:

• المستقبل مهم = The future is important
• مستقبل مهم = usually an important future or an important future/future prospect, depending on context

So the الـ on المستقبل matters a lot here. It helps make the sentence mean the future is important, not just a noun phrase.

Usually, no. In ordinary Arabic writing, you will normally see:

• المستقبل مهم

But the fully marked form is useful for study:

• اَلْمُسْتَقْبَلُ مُهِمٌّ

That version helps you notice:

• the short vowels
• the nominative endings
• the shadda in مهمّ

So for reading real texts, the unvocalized form is normal; for grammar study and pronunciation, the vocalized form is very helpful.