Det saknas en underskrift på sista sidan.

Here det is a dummy subject (also called a formal subject). It does not refer to a real thing like it usually does in English.

Swedish often uses det in sentences like:

• Det finns... = there is / there are
• Det kommer... = there comes / there are coming
• Det saknas... = there is ... missing / ... is missing

So Det saknas en underskrift is structurally similar to There is a signature missing.

In this sentence, saknas means is missing or is lacking.

So:

• Det saknas en underskrift = A signature is missing
• more literally: There lacks a signature

In natural English, we usually would not say there lacks, but Swedish does.

The -s form here is the passive form of sakna.

• sakna = to miss / to lack
• saknas = to be missing

Compare:

• Jag saknar en underskrift. = I am missing a signature / I lack a signature
• Det saknas en underskrift. = A signature is missing

In practice, det saknas is a very common fixed-looking pattern in Swedish, and learners often just memorize it as there is/are ... missing.

No, that would be incorrect.

You need saknas, not saknar, in this sentence.

• Det saknas en underskrift... = correct
• Det saknar en underskrift... = incorrect

If you use saknar, you need a real subject that lacks something:

• Jag saknar en underskrift.
• Blanketten saknar en underskrift. = The form lacks a signature

So saknar needs someone or something that lacks the signature, while det saknas is the impersonal pattern meaning a signature is missing.

Because the sentence is introducing one missing signature, not pointing to a previously identified one.

• en underskrift = a signature
• underskriften = the signature

Here the idea is simply that one signature is absent from the page. So the indefinite form is natural.

If you said underskriften, it would sound like you mean a specific already-known signature:

• Underskriften saknas. = The signature is missing.

That works, but it gives a slightly different focus.

For official documents, underskrift is the normal word for a signature.

• underskrift = signature on a document
• signatur can mean a signature too, but it is often used more broadly for things like an autograph, a stylistic signature, a symbol, initials, or a label/sign-off

So if you are talking about a form, contract, application, or other paperwork, underskrift is usually the safest choice.

Because på is the normal preposition for something written or located on a page.

Examples:

• på sidan = on the page
• på första sidan = on the first page
• på sista sidan = on the last page

A signature is something that appears on the page, so på is the natural preposition.

This is a very common pattern with words like första and sista.

Swedish often says:

• första gången = the first time
• sista sidan = the last page

So på sista sidan is completely normal.

You may also hear på den sista sidan, but that version is often a bit more explicit or emphatic. In many everyday contexts, på sista sidan sounds more natural.

So:

• på sista sidan = normal, common
• på den sista sidan = also possible, often a bit more pointed

Yes, that is grammatical.

Both are correct:

• Det saknas en underskrift på sista sidan.
• En underskrift saknas på sista sidan.

The difference is mostly about focus.

• Det saknas en underskrift... is very natural when you are simply reporting that something is missing.
• En underskrift saknas... puts slightly more attention on en underskrift.

The det saknas version is especially common in everyday and administrative Swedish.

Yes, very much so.

Compare:

• Det finns en underskrift på sista sidan. = There is a signature on the last page.
• Det saknas en underskrift på sista sidan. = There is a signature missing on the last page.

Both use:

• det as a dummy subject
• verb in second position
• the real thing being talked about after the verb

This is a useful pattern to recognize in Swedish.

Then you would change the noun to the plural:

• Det saknas underskrifter på sista sidan. = Signatures are missing on the last page.

If you want to be specific about the number:

• Det saknas två underskrifter på sista sidan. = Two signatures are missing on the last page.

So the singular en underskrift tells you that exactly one signature is missing.