Die graad is belangrik.

Yes. In Afrikaans, die is the definite article the. Unlike many European languages, Afrikaans does not have grammatical gender in its articles, so die is used for singular and plural nouns alike in many contexts.

So:

• die graad = the grade / the degree
• die boeke = the books

This is much simpler than languages that have separate forms like the masculine / feminine / neuter.

Graad can mean several related things depending on context, including:

• grade (as in school results)
• degree (as in an academic qualification)
• degree (as in temperature or angle)
• sometimes level or rank

So in Die graad is belangrik, the exact meaning of graad depends on the wider context. It could refer to a school grade, a qualification, or another kind of degree/level.

This sentence is a simple statement with a subject, the verb is, and a description:

• Die graad = subject
• is = verb
• belangrik = predicate adjective

So the pattern is:

Subject + verb + adjective

That is why it looks very familiar to an English speaker:

• The grade is important.
• Die graad is belangrik.

Afrikaans often has English-like word order in simple main clauses, though more complex sentences can differ.

It has the same meaning here, yes. Is is the present-tense form of to be in Afrikaans, just as in English.

Examples:

• Hy is moeg. = He is tired.
• Dit is goed. = It is good.
• Die graad is belangrik. = The grade is important.

A useful point for learners: Afrikaans verbs are much less heavily conjugated than English verbs.

In Afrikaans, adjectives often stay unchanged when they come after is and describe the subject.

So:

• Die graad is belangrik.
• Die boek is interessant.
• Die kinders is gelukkig.

This is one of the simpler parts of Afrikaans grammar. You do not need to change belangrik here to match gender or number.

Not in this sentence.

After is, you use belangrik:

• Die graad is belangrik.

Before a noun, Afrikaans often uses the inflected form belangrike:

• die belangrike graad = the important grade/degree
• 'n belangrike besluit = an important decision

So the contrast is:

• Die graad is belangrik. = adjective after the verb
• Die belangrike graad ... = adjective before the noun

A rough guide for English speakers:

• graad: sounds roughly like khraht or graaht, with a broad aa sound
• belangrik: roughly buh-LUNG-rik, with the stress usually on lang

A few helpful notes:

• The g in Afrikaans is usually much harsher than the English g. It is often a throaty sound.
• The aa in graad is longer than the a in English grad.
• The final k in belangrik is clearly pronounced.

Pronunciation varies somewhat by accent, but those approximations are a useful starting point.

No, this sentence is singular because graad is singular.

• Die graad is belangrik. = The grade/degree is important.

If you wanted the plural, you would say:

• Die grade is belangrik. = The grades are important.

Notice that:

• graad → grade in the plural
• is often remains is in Afrikaans, even where English would use are

Afrikaans verbs are much simpler than English verbs. The verb wees (to be) often appears as is in present-tense statements for both singular and plural subjects.

So you get:

• Die kind is gelukkig. = The child is happy.
• Die kinders is gelukkig. = The children are happy.

This is very normal in Afrikaans and is one reason many learners find the verb system easier than English.

Yes, but the meaning changes.

• Die graad is belangrik. = The grade / the degree is important
  This refers to a specific grade or degree.
• 'n graad is belangrik. = A grade / a degree is important
  This is more general or indefinite.

So the difference is the same basic one as the versus a/an in English.

It is a complete and natural sentence. Grammatically, nothing is missing.

However, like the English sentence The grade is important, it is a bit context-dependent in real life because graad can mean different things. A speaker would usually know from the surrounding conversation whether it refers to:

• a school grade
• a university degree
• a temperature degree
• a level or rank

So it is fully correct, but the exact interpretation depends on context.