Die toonleer is moeilik.

die is the definite article, usually equivalent to English the.

A useful thing for English speakers to know is that Afrikaans articles are much simpler than in many other European languages:

• die = the
• 'n = a/an

Afrikaans does not change die for grammatical gender, so you do not have to learn separate forms like masculine/feminine/neuter the.

Because die refers to a specific scale: the scale.

If you said 'n toonleer is moeilik, that would mean a scale is difficult, which sounds more general or less specific.

So the difference is:

• Die toonleer is moeilik. = a particular scale is difficult
• 'n toonleer is moeilik. = a scale is difficult

Yes. Afrikaans normally writes compound nouns as one word.

So toonleer is a compound noun, not two separate words. This is very common in Afrikaans:

• musiekboek
• klavierles
• toonleer

English often separates things more easily, but Afrikaans strongly prefers one-word compounds.

Afrikaans verbs are much less complicated than English verbs. In the present tense, is is used very broadly:

• ek is
• jy is
• hy/sy is
• ons is
• hulle is

So unlike English, you do not have to remember different forms like am, is, and are.

Because moeilik is being used after the verb is. In that position, Afrikaans adjectives usually stay in their basic form.

So:

• Die toonleer is moeilik.
  Here moeilik is a predicate adjective after is.

But:

• 'n moeilike toonleer
  Here the adjective comes before the noun, so it often takes -e.

This is a very important pattern in Afrikaans:

• after is → usually no -e
• before a noun → often -e

In this sentence, yes. It follows a very familiar pattern:

• Die toonleer = subject
• is = verb
• moeilik = complement/adjective

So it is basically: subject + verb + adjective

That said, Afrikaans is generally a verb-second language in main clauses. In a simple sentence like this, that looks similar to English, but in other sentences the word order can shift.

For example:

• Vandag is die toonleer moeilik.

The verb is still comes early in the sentence.

In standard Afrikaans, die is usually pronounced roughly like dee, not like the English word die.

That is one of the first pronunciation traps for English speakers.

Also, in moeilik, the spelling oe usually sounds like the vowel in food. So moeilik begins with a sound roughly like moo-.

You would usually say:

Die toonleer is nie moeilik nie.

This shows a very common Afrikaans pattern: double nie.

A simple way to think of it is:

• first nie comes after the verb or main idea
• second nie comes near the end of the clause

So:

• Die toonleer is moeilik.
• Die toonleer is nie moeilik nie.

This double-negation pattern is one of the most important things to get used to in Afrikaans.