Siffran i boken är svår att förstå.

Siffra is the indefinite form (like a digit / a figure).
Siffran is the definite form (like the digit / the figure).

In Swedish, definiteness is usually shown with an ending, not a separate word:

• en siffra = a digit
• siffran = the digit

So Siffran i boken means “The digit in the book”, not “A digit in the book.”

Swedish normally puts “the” at the end of the noun as a suffix.

For en-words (common gender):

• en bok = a book → boken = the book
• en siffra = a digit → siffran = the digit

For ett-words (neuter):

• ett hus = a house → huset = the house

So siffran already means “the digit”; you don’t add another word for “the” in front of it.

They overlap, but they’re not identical:

• siffra = a digit / numeral symbol (0–9, or sometimes a single written figure)
  • ex: Siffran 7 är fel. = The digit 7 is wrong.
• nummer = a number used as a label (phone number, house number, player number, etc.)
  • ex: Vilket är ditt telefonnummer?
• tal = a number as a quantity (mathematical value) or speech (depending on context)
  • ex (number): Ett stort tal = a large number
  • ex (speech): Hon höll ett tal. = She gave a speech.

In Siffran i boken är svår att förstå, siffran suggests a particular written figure or numeric expression that is difficult to interpret.

i generally means “in” (inside something), while på means “on” (on the surface of something).

• i boken = in the book (inside its contents, on the pages)
• på boken = on the book (on the cover, physically on top of the book)

Here we’re talking about a digit that appears in the content of the book, so i boken is correct.

Same definiteness rule as with siffran:

• en bok = a book (indefinite)
• boken = the book (definite)

The English sentence is “The digit in the book is hard to understand.”
Since English uses “the”, Swedish also needs the definite form: boken, not bok.

The pattern adjective + att + infinitive is very common in Swedish:

• är svår att förstå = is hard to understand
• literally: is difficult to understand

Here:

• svår = difficult / hard
• att förstå = to understand

Att marks the infinitive (like “to” in “to understand”), and förstå stays in its base form.
You don’t need a passive -s ending (förstås) here; that would change the meaning to something like “is understood” (passive voice), which is not intended.

Here, att is the infinitive marker, equivalent to “to” before a verb in English:

• att förstå = to understand
• att läsa = to read
• att skriva = to write

It is not the same att as “that” in sentences like “I think that…”.
That att introduces a clause (and is often omitted in spoken Swedish), while this att simply forms the infinitive.

Adjectives in Swedish agree with the gender and number of the noun they describe.

• siffra is an en-word (common gender).
• The en-word form of the adjective is svår.
• The ett-word form would be svårt.
• The plural form would be svåra.

So:

• Siffran är svår. = The digit is difficult.
• Det här talet är svårt. = This number (ett tal) is difficult.
• Siffrorna är svåra. = The digits are difficult.

Since siffran is an en-word, you must use svår.

Yes, that’s a very natural sentence:

• Siffran i boken är svår att förstå.
  
  • Focus is on the digit itself: “The digit in the book is hard to understand.”
• Det är svårt att förstå siffran i boken.
  
  • More general statement about the situation: “It is hard to understand the digit in the book.”

Both are correct; the nuance is similar to English:

• The digit is hard to understand.
  vs.
• It’s hard to understand the digit.

Swedish typically uses the structure:

[subject] + är + [adjective] + att + [infinitive]

So:

• Siffran i boken (subject)
• är (verb)
• svår (adjective)
• att förstå (infinitive phrase)

You don’t say *är att förstå svår; that would sound incorrect and unnatural. The adjective comes directly after the verb and before att + verb, just like in English: “is hard to understand”, not “is to understand hard”.

Siffra is an en-word (common gender):

• en siffra = a digit
• siffran = the digit
• den här siffran = this digit

That’s why you see:

• svår (en-word form)
  and not svårt (ett-word form) in the sentence.