Dit is de achtste vraag.

Yes. Dit means this.

In Dit is de achtste vraag, dit is a demonstrative pronoun: it points to something and stands on its own, like English this in This is....

A useful comparison:

• Dit is de achtste vraag. = This is the eighth question.
• Deze vraag is de achtste. = This question is the eighth.

So dit is used when this stands by itself, while deze is usually used directly before a de-word noun.

Because vraag is a de-word in Dutch.

Dutch nouns take either:

• de
• het

The noun vraag = question is a de noun, so you say:

• de vraag
• de achtste vraag

Not:

• het vraag
• het achtste vraag

This is something you usually have to learn with each noun.

Achtste means eighth. It is the ordinal form of acht (eight).

So:

• acht = eight
• achtste = eighth

Dutch ordinal numbers are often made by adding -de or -ste, though there are some patterns and irregular forms you just get used to.

Examples:

• eerste = first
• tweede = second
• derde = third
• vierde = fourth
• vijfde = fifth
• zesde = sixth
• zevende = seventh
• achtste = eighth

Because it is being used as an adjective before a noun.

In de achtste vraag, achtste describes vraag, so it behaves like an adjective. In Dutch, adjectives and ordinal numbers before a noun often take -e.

Compare:

• de achtste vraag = the eighth question
• de eerste dag = the first day
• de laatste trein = the last train

So the -e here is normal.

Because Dutch normally needs an article here, just like English does.

You are talking about a specific numbered item: the eighth question, not just eighth question in general.

So:

• Dit is de achtste vraag. = correct
• Dit is achtste vraag. = incorrect

The article de makes the phrase definite and complete.

This is standard Dutch main-clause word order.

The structure is:

• Dit = subject
• is = verb
• de achtste vraag = complement

So it follows the normal pattern:

subject + verb + rest

Just like:

• Dit is mijn boek.
• Dat is een probleem.

Dutch is a verb-second language, so in a normal main clause, the conjugated verb usually comes in the second position. Here, is is in second position.

Usually, no—not in the same natural way.

If you mean This is the eighth question, Dutch normally uses dit:

• Dit is de achtste vraag.

Deze is more naturally used directly before a noun:

• deze vraag = this question

So:

• Deze vraag is de achtste. = correct
• Dit is de achtste vraag. = correct
• Deze is de achtste vraag. = possible only in a very specific context, but not the usual choice here

For learners, the safest rule is:

• use dit/dat when it stands alone
• use deze/die before de-words
• use dit/dat before het-words

This word can be tricky because of the consonant cluster.

A rough guide:

• acht has the Dutch ch sound, like the harsh throat sound in Scottish loch
• then comes -ste

So achtste is roughly AKH-tstuh or AKH-stuh, depending on how clearly someone pronounces the middle sounds.

The hardest part is the chtst cluster. In careful speech, all the sounds are there, but in natural speech they may blend a bit.

Yes.

• vraag = question
• vragen = to ask

They are clearly related, like:

• a question
• to ask

Examples:

• Ik stel een vraag. = I ask a question / I pose a question.
• Ik vraag iets. = I ask something.

So in this sentence, vraag is the noun meaning question.

No. It is a statement.

Even though vraag means question, the whole sentence is just telling you something:

• Dit is de achtste vraag. = This is the eighth question.

It does not have question word order, and in writing it ends with a period, not a question mark.

So the sentence is about a question, but it is not itself a question.