Dit is my gewoonte om eers die venster oop te maak.

Because Afrikaans is using a structure equivalent to English It is my habit to ... rather than I ....

So:

Dit is my gewoonte om eers die venster oop te maak.
literally follows the pattern It is my habit to first open the window.

This is a very normal way to talk about habits in Afrikaans. If you wanted a more direct I sentence, you could also say:

Ek maak gewoonlik eers die venster oop.
= I usually open the window first.

The original sentence sounds a bit more like a statement about a personal routine or custom.

Gewoonte means habit, custom, or usual practice.

In this sentence, my gewoonte means my habit or what I normally do.

A useful pattern is:

Dit is my gewoonte om ...
= It is my habit to ...

So the sentence is not just describing one action; it is describing something the speaker typically does.

Yes. My is the possessive word meaning my.

Examples:

• my huis = my house
• my boek = my book
• my gewoonte = my habit

Afrikaans possessives are much simpler than in many other languages:

• they do not change for gender
• they do not change for case

So my stays my before any noun.

Om ... te is a very common Afrikaans infinitive structure. It often corresponds to English to + verb.

Here:

om eers die venster oop te maak
= to first open the window

So:

• om introduces the infinitive clause
• te marks the infinitive verb

You will often see this after nouns, adjectives, and certain verbs.

For example:

• Dit is maklik om te leer. = It is easy to learn.
• Ek probeer om te help. = I try to help.

In your sentence, the infinitive clause explains what the habit is.

Because oopmaak is a separable verb.

The full verb is: oopmaak = to open

It is made of:

• oop = open
• maak = make

In Afrikaans, separable verbs often split in certain sentence patterns. With te, the te usually goes between the two parts:

• oopmaak → oop te maak
• aanskakel → aan te skakel
• toemaak → toe te maak

So oop te maak is exactly what you expect in an om ... te clause.

Because in an om ... te infinitive clause, the verb normally comes at the end.

So the order is roughly:

om + other information + verb at the end

In this sentence:

• om
• eers
• die venster
• oop te maak

The object die venster comes before the infinitive verb phrase, and the final verbal part maak appears at the end. This is very typical Afrikaans word order.

Here eers means first or before anything else.

So: eers die venster oop te maak
means to open the window first

A common learner mistake is to think eers always means only or just now, but its meaning depends on context. In this sentence, it clearly means sequence: this action happens before other actions.

Die is the definite article, meaning the.
’n is the indefinite article, meaning a/an.

So:

• die venster = the window
• ’n venster = a window

The sentence uses die venster because it refers to a specific window, or at least a window understood from the situation.

Also, Afrikaans is simpler than English in one useful way:

• die is used for the with both singular and plural nouns
• there is no grammatical gender to worry about

Examples:

• die boek = the book
• die boeke = the books

Yes, Afrikaans has some flexibility here, though the emphasis can change slightly.

Your sentence: Dit is my gewoonte om eers die venster oop te maak.

Another possible version: Dit is my gewoonte om die venster eers oop te maak.

Both are understandable. The original version sounds very natural and emphasizes first a little earlier. In everyday use, adverbs like eers can sometimes move, but not completely freely, so it is best to learn the original pattern first.

Yes, oopmaak is the normal everyday Afrikaans verb for open something.

Examples:

• Maak die deur oop. = Open the door.
• Sy maak die venster oop. = She opens the window.

Notice that in a main clause, the parts are often separated:

• maak ... oop

But the dictionary form is:

• oopmaak

So in your sentence you are seeing the same verb inside an infinitive structure:

• oop te maak

Yes. In normal speech, Dit is is very often contracted to Dis.

So you may hear:

Dis my gewoonte om eers die venster oop te maak.

This means exactly the same thing.
The full form Dit is is perfectly correct, and the shortened form Dis is very common in everyday Afrikaans.