Ek maak die tenk vol.

That is exactly the structure: maak ... vol = make ... full, which is a very natural Afrikaans way to say fill up.

So:

• Ek = I
• maak = make
• die tenk = the tank
• vol = full

Together, Ek maak die tenk vol means I fill the tank / I’m filling the tank up.

Because Afrikaans often uses the pattern:

maak + object + adjective

So you are making something into a certain state.

Examples:

• Ek maak die deur oop. = I open the door.
  Literally: I make the door open.
• Ek maak die kamer skoon. = I clean the room.
  Literally: I make the room clean.
• Ek maak die tenk vol. = I fill the tank.
  Literally: I make the tank full.

So vol comes after die tenk because it describes the result of what happens to the tank.

Yes. Vul means fill, so Ek vul die tenk is also correct.

The difference is mostly style and wording:

• Ek vul die tenk = I fill the tank
• Ek maak die tenk vol = I fill the tank up / I make the tank full

In everyday speech, vol maak is very common and idiomatic.

No. In Afrikaans, verbs usually do not change form for person in the present tense.

So you get:

• Ek maak = I make
• Jy maak = you make
• Hy maak = he makes
• Ons maak = we make

This is much simpler than English.

Die means the.

Afrikaans uses die as the definite article for nouns. Unlike languages such as German or Dutch, Afrikaans does not have grammatical gender for articles, so you do not need to learn different forms like masculine/feminine/neuter.

So:

• die tenk = the tank

Not always. Tenk can mean tank in a general sense: a fuel tank, water tank, storage tank, and so on.

In this sentence, context usually tells you what kind of tank it is. If the meaning needs to be clearer, Afrikaans can use a more specific word, such as:

• petroltenk = petrol/gas tank
• watertenk = water tank

Yes. The simple present in Afrikaans often covers both ideas, depending on context.

So Ek maak die tenk vol can mean:

• I fill the tank
• I am filling the tank
• sometimes even I’m going to fill the tank in the right context

If you want to emphasize that the action is in progress, Afrikaans can use a longer structure such as:

• Ek is besig om die tenk vol te maak.
  = I am busy filling the tank.

Yes. Vol means full, and it can be used in other normal adjective sentences too.

For example:

• Die tenk is vol. = The tank is full.
• Die glas is vol. = The glass is full.

In Ek maak die tenk vol, the adjective vol describes the result of the action.

Afrikaans usually uses a double nie in negative sentences.

So:

• Ek maak die tenk vol. = I fill the tank.
• Ek maak nie die tenk vol nie. = I do not fill the tank.

That double negative structure is a very important feature of Afrikaans.

In a yes/no question, Afrikaans usually puts the verb first:

• Maak ek die tenk vol? = Am I filling the tank? / Do I fill the tank?

This verb-first pattern is very common in Afrikaans questions.

A rough English-friendly pronunciation is:

Ek mahk dee tenk foll

A few helpful notes:

• Ek sounds a bit like eck
• maak has a long aa sound
• die here sounds like dee
• tenk is close to English tank, but with a shorter vowel
• vol sounds like foll

The g sound is not a problem here, because this sentence does not contain one.