Beslutet är viktigt.

The ending -et is the definite article attached to the noun. Swedish often puts the at the end of the noun instead of using a separate word before it.

• ett beslut = a decision
• beslutet = the decision

So beslutet is the definite form of beslut.

Because in Swedish, definiteness is usually shown by a suffix on the noun.

In English:

• the decision

In Swedish:

• beslutet

That -et already means the, so you do not normally add another article in a simple sentence like this.

Är is the present tense of the verb vara, which means to be.

So:

• är = is / are / am depending on context

In this sentence:

• Beslutet är viktigt = The decision is important

Swedish verbs do not change depending on the subject the way English verbs sometimes do. Är stays the same whether the subject is singular or plural.

Because beslut is an ett-word (a neuter noun), and adjectives must agree with the noun they describe.

The adjective is viktig = important. Its forms include:

• en viktig bok = an important book
• ett viktigt beslut = an important decision
• boken är viktig = the book is important
• beslutet är viktigt = the decision is important

So the -t in viktigt matches the fact that beslut is an ett-word.

You learn it as part of the noun:

• ett beslut = a decision

In Swedish, every noun has a grammatical gender, usually called en or ett. There is no perfect rule that lets you guess every time, so it is best to memorize nouns together with their article:

• en bok
• ett beslut

Knowing that it is ett beslut helps you form both:

• the definite form: beslutet
• the correct adjective form: viktigt

The basic word order is:

subject + verb + adjective/complement

So here:

• Beslutet = subject
• är = verb
• viktigt = complement

This is very similar to English:

• The decision is important

It is a simple statement, so the word order is straightforward.

Yes, but it means something slightly different.

• Beslutet är viktigt = The decision is important
  This refers to a specific decision.

• Ett beslut är viktigt = A decision is important
  This is more general or indefinite.

So both are correct, but they are not used in exactly the same situation.

A simple approximate pronunciation for an English speaker is:

beh-SLOO-tet air VIK-tigt

A few notes:

• beslutet has the stress mainly on slu
• är sounds somewhat like air, though not exactly
• viktigt ends with a light -tigt sound; in careful speech you may hear the g, but in natural speech it can sound softer

If you want to sound natural, focus especially on:

• clear long vowel in är
• stress on slu in beslutet

No. Here viktigt is an adjective, not an adverb.

It describes the subject beslutet:

• The decision is important

After är, Swedish often uses an adjective to describe the subject:

• Huset är stort = The house is big
• Beslutet är viktigt = The decision is important

So viktigt is a predicate adjective agreeing with beslutet.

They often do, depending on the noun.

A simplified pattern is:

• en-word: viktig
• ett-word: viktigt
• plural / definite plural: viktiga

Examples:

• en viktig fråga = an important question
• ett viktigt beslut = an important decision
• viktiga beslut = important decisions

In your sentence, the adjective is in the ett form because beslut is an ett-word.