A mentesség fontos.

In Hungarian, the verb van (to be) is usually left out in the present tense, third person singular when you have a sentence like X is Y.

So:

• A mentesség fontos. = The exemption is important.

But in other tenses, or with other persons, the verb appears:

• A mentesség fontos volt. = The exemption was important.
• A mentesség fontos lesz. = The exemption will be important.

A is the Hungarian definite article, meaning the.

Hungarian has two forms of the definite article:

• a before a consonant sound
• az before a vowel sound

Since mentesség starts with m, a consonant, you say:

• a mentesség

If the next word began with a vowel, you would use az instead.

Hungarian often uses the definite article with abstract nouns or general concepts in places where English may leave the article out.

So A mentesség fontos. can still be a general statement, not only a very specific the exemption in a narrow context.

This is a common difference between English and Hungarian. Hungarian often sounds more natural with the article in sentences about broad ideas.

Mentesség is a noun.

It is built from:

• mentes = free from, exempt
• -ség = a noun-forming suffix, often like English -ness or sometimes something more abstract like state/status/condition

So mentesség means something like:

• exemption
• immunity
• the state of being free from something

The exact English translation depends on context, but grammatically it is a singular noun here.

Because fontos is not directly modifying the noun here; it is the predicate adjective.

Compare:

• A mentesség fontos. = The exemption is important.
• fontos mentesség = important exemption

In the sentence you gave, mentesség is the subject, and fontos tells us something about it.

It is singular and in the nominative case.

Why?

• There is no plural ending, so it is singular.
• It is the subject of the sentence, so the basic nominative form is used.

If it were plural, it would look different:

• A mentességek fontosak. = The exemptions are important.

Notice that in the plural, fontos also becomes fontosak.

Yes. Fontos a mentesség is also possible.

Both sentences are grammatical, but the word order changes the focus or emphasis a bit:

• A mentesség fontos.
  The topic is the exemption, and you say something about it.

• Fontos a mentesség.
  The idea of importance is foregrounded more.

Hungarian word order is flexible, but it is not random; it often reflects emphasis and information structure.

A few helpful pronunciation points:

• Stress is on the first syllable: MEN-tes-ség
• Hungarian s sounds like English sh
• ss is a long version of that same sound
• é is a long e sound

So mentesség is pronounced approximately [ˈmɛntɛʃːeːg].

Also, in the whole sentence:

• fontos ends with s, so that final sound is also sh, not s

So fontos is pronounced roughly [ˈfontoʃ].