Dit is de elfde vraag.

Dit means this.

In Dit is de elfde vraag, dit is a demonstrative pronoun: it stands on its own and refers to this thing / this one / this.

You would use deze when it comes directly before a de-word noun:

• deze vraag = this question

So compare:

• Dit is de elfde vraag. = This is the eleventh question.
• Deze vraag is de elfde. = This question is the eleventh one.

A common learner mistake is to try to say Dit vraag, but that is not correct.

Because vraag is a de-word in Dutch:

• de vraag = the question

Dutch nouns take either de or het as their definite article, and this has to be learned with the noun.

So:

• de vraag
• een vraag

Since vraag is a de-word, the phrase must be:

• de elfde vraag

not:

• het elfde vraag

Elfde means eleventh. It is the ordinal number formed from elf = eleven.

So:

• elf = eleven
• elfde = eleventh

Some useful nearby examples:

• tiende = tenth
• elfde = eleventh
• twaalfde = twelfth

In this sentence, elfde describes vraag, so it works like eleventh in English:

• de elfde vraag = the eleventh question

Because it comes before a noun and is used in a definite noun phrase:

• de elfde vraag

In Dutch, adjectives and ordinal numbers often take -e before nouns, especially after de or het.

That is why you get:

• de elfde vraag
• de tiende les
• het eerste boek

So the -e here is normal grammar, not part of the number by accident.

This is a very basic Dutch sentence pattern:

• subject + verb + complement

Here:

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

So literally:

• This + is + the eleventh question

This matches English word order very closely, which makes this sentence relatively easy for English speakers.

Yes, that can be correct in the right context, but it is less neutral than Dit is de elfde vraag.

• Dit is de elfde vraag. = This is the eleventh question.
• Deze is de elfde vraag. = This one is the eleventh question.

Deze often has a stronger sense of this one here, especially when contrasting with other items.

Also remember:

• deze vraag = this question
• dit boek = this book

But when the word stands alone, Dutch often uses dit very naturally, especially when introducing or identifying something.

A rough English-friendly guide would be:

• Dit ≈ dit
• is ≈ iss
• de ≈ duh or a very short de
• elfde ≈ ELF-duh
• vraag ≈ something like vrahkh, with a throaty Dutch g

A few pronunciation notes:

• v in Dutch is often between English v and f
• aa in vraag is a long vowel
• the g in vraag is the hard Dutch throat sound

So the hardest word here for many English speakers is vraag.

Yes, sometimes.

• Dit is de elfde vraag is the normal way to say This is the eleventh question.
• Dit is vraag elf means more literally This is question eleven.

The second version is common when numbering things in a practical way, for example:

• exercises
• forms
• quiz questions
• survey items

So both can be natural, but they are used slightly differently:

• de elfde vraag = the eleventh question
• vraag elf = question eleven

Yes. Is is the third-person singular form of zijn, which means to be.

So:

• ik ben = I am
• jij bent = you are
• hij/zij/het is = he/she/it is
• dit is = this is

In this sentence, is works exactly the way an English speaker would expect.

Because dit is already functioning as the subject by itself. It does not need an article.

Compare:

• Dit is de elfde vraag. = This is the eleventh question.
• Deze vraag is de elfde. = This question is the eleventh.

In the first sentence, dit already means this, so adding de or het before it would be wrong.

Vraag is singular, and vragen is plural.

• de vraag = the question
• de vragen = the questions

So:

• Dit is de elfde vraag. = This is the eleventh question.
• Dit zijn de elfde vragen. would mean These are the eleventh questions, which is much less likely in normal use.

In your sentence, everything is singular:

• dit
• is
• de elfde vraag