Die maandelikse fooi word na gelang van die kontraktydperk bepaal.

Because die is the normal definite article in Afrikaans, and it is used for the in both singular and plural.

So here:

• Die maandelikse fooi = the monthly fee
• die kontraktydperk = the contract period / the term of the contract

Unlike English, Afrikaans does not change the article for gender, and it does not have separate forms like the vs a/an in the same way English learners might expect from other languages.
If it were indefinite, you would often see ’n instead:

• ’n maandelikse fooi = a monthly fee

Maandelikse means monthly.

It comes from maandeliks, which is the basic adverb/adjective form meaning monthly or on a monthly basis. When many adjectives come before a noun in Afrikaans, they take an -e ending.

So:

• maandeliks = monthly
• maandelikse fooi = monthly fee

This is very common in Afrikaans:

• klein → ’n klein huis
• groot → ’n groot probleem
• maandeliks → maandelikse betaling / maandelikse fooi

So the -e here is an adjective ending used before the noun fooi.

Fooi usually means fee, charge, or sometimes tuition/levy, depending on context.

In this sentence, maandelikse fooi is best understood as:

• monthly fee
• monthly charge
• possibly monthly payment, depending on the situation

Be careful not to confuse it with English tip. In some contexts, Afrikaans fooi can also refer to a tip or gratuity, but in business or contract language it very often means fee.

This is the present passive in Afrikaans.

• word = is/gets (used to form the passive)
• bepaal = determined

So:

• word bepaal = is determined / gets determined

The sentence is passive because the focus is on the monthly fee, not on who determines it.

Compare:

• Die maandelikse fooi word bepaal ... = The monthly fee is determined ...
• Die maatskappy bepaal die maandelikse fooi ... = The company determines the monthly fee ...

Afrikaans often uses word for a present passive and is for a completed passive:

• word bepaal = is being determined / is determined
• is bepaal = has been determined / was determined, depending on context

Not really in this sentence, although it is the same word.

Afrikaans word can mean:

1. becomes

   • Hy word moeg. = He becomes tired.

2. a passive helper verb, like is/gets in English passive structures

   • Die fooi word bepaal. = The fee is determined.

So here, word is not saying the fee becomes determined in a literal change-of-state sense. It is simply forming the passive.

Na gelang van is a set expression meaning:

• according to
• depending on
• in accordance with
• based on

In this sentence, it means the monthly fee is determined depending on the contract period.

You should learn na gelang van as a whole phrase rather than translating each word separately. Word-for-word, it is not very transparent for an English speaker.

Examples:

• Pryse wissel na gelang van aanvraag.
  Prices vary depending on demand.

• Die bedrag word na gelang van jou inkomste bereken.
  The amount is calculated according to your income.

Because na gelang van is the fixed expression. The van belongs to the phrase.

So you should think of it as:

• na gelang van iets = depending on something

Examples:

• na gelang van die kontraktydperk
• na gelang van ouderdom
• na gelang van beskikbaarheid

It is similar to learning an English phrase like according to or depending on. You generally do not change the preposition.

Yes, it is one word because Afrikaans, like Dutch and German, very often forms compound nouns as a single written word.

kontraktydperk is made up of:

• kontrak = contract
• tydperk = period / term

So kontraktydperk means:

• contract period
• contract term
• duration of the contract

This kind of compounding is very common in Afrikaans:

• motorhuis = garage
• skoolhoof = school principal
• werknemerbelasting = employee tax

Because in Afrikaans main clauses, the finite verb usually comes early, but the main verb often goes to the end when there is an auxiliary or passive construction.

Here the finite verb is word, and the main verb is bepaal:

• Die maandelikse fooi
• word
• na gelang van die kontraktydperk
• bepaal

This is a very common Afrikaans pattern:

• Die werk word môre gedoen.
  The work is done tomorrow.

• Die brief is reeds geskryf.
  The letter has already been written.

So bepaal goes to the end because it is the main verb in a passive structure with word.

Yes, but the emphasis would change.

A possible active version would be:

• Die kontraktydperk bepaal die maandelikse fooi.

That means The contract term determines the monthly fee.

Another possibility:

• Ons bepaal die maandelikse fooi na gelang van die kontraktydperk.
  We determine the monthly fee according to the contract term.

The passive version sounds more formal and impersonal, which is very common in contracts, policies, and business writing.

It is actually a very useful verb. Bepaal commonly means:

• determine
• decide
• set
• establish

Examples:

• Die prys word bepaal deur vraag en aanbod.
  The price is determined by supply and demand.

• Ons moet die datum bepaal.
  We must determine/set the date.

• Die reëls bepaal wat toegelaat word.
  The rules determine what is allowed.

So in your sentence, bepaal is a standard formal verb meaning determined or set.

It sounds fairly formal and would fit well in:

• contracts
• service agreements
• pricing information
• official documents
• websites with terms and conditions

The main things that make it sound formal are:

• the passive structure word ... bepaal
• the expression na gelang van
• the compound noun kontraktydperk

In everyday spoken Afrikaans, someone might say something a bit simpler, depending on context, such as:

• Die maandelikse fooi hang van die kontraktydperk af.

That means roughly The monthly fee depends on the contract term.

A simpler, more conversational version would be:

• Die maandelikse fooi hang van die kontraktydperk af.

This uses hang van ... af, which is a very common way to say depends on.

Compare the two:

• Die maandelikse fooi word na gelang van die kontraktydperk bepaal.
  More formal, official, written style.

• Die maandelikse fooi hang van die kontraktydperk af.
  More direct and natural in everyday language.

Both are correct, but they suit different situations.