Is jy seker dat die deur al oop is?

Because this is a yes/no question.

In Afrikaans, just like in English, the verb often comes before the subject in a direct question:

• Jy is seker. = You are sure.
• Is jy seker? = Are you sure?

So Is jy seker ...? is the normal question order.

Seker means sure or certain.

So:

• Ek is seker. = I am sure.
• Is jy seker? = Are you sure?

It is a very common word in everyday Afrikaans.

Dat means that.

It introduces the clause die deur al oop is, which gives the content of what someone is sure about:

• Is jy seker ... = Are you sure ...
• dat die deur al oop is? = that the door is already open?

So dat works very much like English that.

Sometimes, yes, especially in informal speech, but learners should usually keep it.

So you may hear:

• Is jy seker die deur is al oop?

But Is jy seker dat die deur al oop is? is clear and standard, and it helps show the sentence structure more clearly.

Because after dat, Afrikaans usually sends the finite verb to the end of the clause.

Compare:

• Main clause: Die deur is al oop.
• Clause after dat: dat die deur al oop is

This is one of the most important word-order patterns in Afrikaans:

• in a normal main clause, the verb comes early
• in a dat clause, the verb often comes at the end

That is why you get oop is, not is oop.

Here al means already.

So:

• die deur is oop = the door is open
• die deur is al oop = the door is already open

In this sentence, al adds the idea that the door has become open earlier than expected, or that this may be surprising.

In a sentence with is plus an adjective, al usually goes before the adjective:

• Die deur is al oop.

And in the dat clause:

• dat die deur al oop is

So the pattern here is:

• subject: die deur
• adverb: al
• adjective: oop
• verb at the end: is

No, Afrikaans does not use grammatical gender the way German does.

Die is the normal definite article for singular and plural nouns:

• die deur = the door
• die huis = the house
• die kinders = the children

So you do not have to learn separate words like the masculine, the feminine, and the neuter article. That makes Afrikaans easier than some related languages.

Yes. They are cognates, meaning they come from the same historical source.

• English: door
• Afrikaans: deur

Even though they look a little different, they are clearly related in meaning.

Because Afrikaans normally uses the adjective oop for the state open.

So:

• Die deur is oop. = The door is open.

This describes the condition of the door, not the action of opening it.

A form like geopen is much less natural in everyday speech for this meaning. In ordinary Afrikaans, oop is the normal choice.

The basic statement is:

• Die deur is al oop. = The door is already open.

When that statement becomes a clause after dat, the word order changes:

• dat die deur al oop is

So a useful learning step is:

1. Start with the main clause: Die deur is al oop.
2. Put it after dat
3. Move is to the end: dat die deur al oop is

That is not the best standard structure.

If you use dat, then:

• Is jy seker dat die deur al oop is?

If you leave out dat, the more natural version is usually:

• Is jy seker die deur is al oop?

So:

• with dat -> subordinate-clause order: ... die deur al oop is
• without dat -> main-clause order is commonly used: ... die deur is al oop

Yes. Jy is the normal singular informal you.

Examples:

• Jy is laat. = You are late.
• Is jy reg? = Are you okay?

Afrikaans also has:

• u for formal you
• julle for plural you

But in everyday speech, jy is extremely common.

A statement version would be:

• Jy is seker dat die deur al oop is. = You are sure that the door is already open.

So the only big change is the main-clause word order at the beginning:

• statement: Jy is seker ...
• question: Is jy seker ...