Die deur is oop.

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

So:

• die deur = the door
• 'n deur = a door

In a normal full sentence, Afrikaans usually uses the article when English would use one. Saying just Deur is oop would sound incomplete in ordinary speech.

No. Afrikaans does not have grammatical gender in its articles the way Dutch or German does.

Die is used for:

• masculine nouns
• feminine nouns
• neuter nouns
• singular nouns
• plural nouns

So die is a very flexible word. That makes it easier for English speakers.

Is is the present-tense form of to be.

In Afrikaans, this form stays the same with all persons:

• ek is = I am
• jy is = you are
• hy/sy is = he/she is
• ons is = we are
• hulle is = they are

So in Die deur is oop, is simply means is.

Because oop is describing the state of the door, and it comes after the linking verb is, just like in English:

• The door is open
• Die deur is oop

Here, oop is a predicate adjective.

Compare:

• Die deur is oop = The door is open
• die oop deur = the open door

So when the adjective is part of the description after is, it comes after the verb.

In this sentence, oop is an adjective. It describes the condition of the door.

So Die deur is oop means the door is in an open state.

If you want the action to open something, Afrikaans usually uses a different structure, for example:

• Maak die deur oop = Open the door
• Hy maak die deur oop = He opens the door

So English open can be both an adjective and a verb, but Afrikaans often separates those ideas more clearly.

Yes. Deur can also be a preposition meaning through or sometimes by.

For example:

• Hy loop deur die park = He walks through the park

But in Die deur is oop, it clearly means the noun door, because:

• it has the article die
• it is the subject of the sentence
• the rest of the sentence describes its state

So the grammar tells you which meaning is intended.

Afrikaans normally uses a double negative:

Die deur is nie oop nie.

This literally looks like:

• nie ... nie

That is the normal Afrikaans negative pattern in many sentences. English speakers often forget the second nie, but it is important.

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

Is die deur oop?

That is very similar to English:

• The door is open.
• Is the door open?

So the main change is word order.

A rough English-friendly guide is:

• die ≈ dee
• deur has a vowel that does not match standard English exactly; it is not the same as English door
• is ≈ iss
• oop ≈ ohp

A very rough overall approximation could be:

dee duhr iss ohp

But that is only approximate. The vowel in deur is especially worth listening to from native speakers, because English does not have an exact equivalent in most accents.

Yes, in this simple statement it is very similar:

• Die deur = subject
• is = verb
• oop = complement/adjective

So it follows the same basic pattern as The door is open.

This makes sentences like this one quite approachable for English speakers, even though Afrikaans has other word-order patterns in more complex sentences.