La centratura è più difficile di quanto credevo.

Centratura can mean centering, alignment, or the act of getting something properly centered, depending on context.

So in this sentence, it probably means something like:

• Centering is more difficult than I thought
• Getting it centered is more difficult than I thought
• Alignment is more difficult than I thought

The exact English choice depends on what is being talked about: a wheel, a photo, a design element, a machine part, and so on.

Because centratura is a feminine singular noun in Italian, and it often takes the definite article la.

• la centratura = the centering / the alignment

Italian uses definite articles more often than English does. In English, you might simply say Centering is difficult, but Italian very naturally says La centratura è difficile.

È is the third-person singular of essere (to be).

So:

• La centratura è... = The centering is...

It agrees with la centratura, which is singular.
If the noun were plural, you would use sono instead:

• Le centrature sono difficili = The centerings/alignments are difficult

Also note the accent: è with an accent means is. This is different from e, which means and.

Più difficile means more difficult.

This is the normal Italian way to make the comparative of an adjective:

• difficile = difficult
• più difficile = more difficult

So the pattern is:

• più + adjective

Examples:

• più facile = easier / more easy
• più interessante = more interesting
• più complicato = more complicated

In your sentence:

• La centratura è più difficile...
  = Centering is more difficult...

In comparisons like this, Italian often uses più ... di quanto + verb to mean more ... than ... when the second part is a whole clause.

So:

• più difficile di quanto credevo
  = more difficult than I thought

Literally, di quanto is something like than what / than how much in this kind of structure, but you should learn it as a set pattern.

A useful rule:

• più + adjective/adverb + di + noun/pronoun
  • più difficile di questo = more difficult than this
• più + adjective/adverb + di quanto + clause
  • più difficile di quanto credevo = more difficult than I thought

Usually, di quanto credevo is the more standard and natural choice in a sentence like this.

• La centratura è più difficile di quanto credevo.
  = very natural

Using che after a comparative can happen in some structures, but here di quanto is the pattern learners should remember.

So if you want to say more difficult than I thought, di quanto credevo is the safest and most idiomatic option.

Credevo is the imperfect tense of credere (to believe / to think), and here it means I thought or I used to think in the sense of what you believed before realizing the truth.

So:

• credo = I believe / I think
• credevo = I thought / I believed

The sentence contrasts past expectation with present reality:

• Now: it is more difficult
• Before: I thought it would be less difficult

That is why credevo fits so well.

Italian often drops subject pronouns because the verb ending already shows who the subject is.

• credevo = I thought
• The ending -evo tells you it is io

So:

• di quanto credevo = than I thought

You could say di quanto io credevo, but that would usually add emphasis or contrast. In normal speech and writing, leaving out io is more natural.

Yes, often you could.

• credevo = I believed / I thought
• pensavo = I thought

In many everyday contexts, both work:

• La centratura è più difficile di quanto credevo.
• La centratura è più difficile di quanto pensavo.

Both are natural.
If there is a slight difference, pensavo often sounds a bit more like I had the impression / I thought, while credevo can sound a bit more like I believed. But in this sentence, the difference is small.

Because difficile is an adjective that has the same singular form for both masculine and feminine nouns.

So you get:

• il compito è difficile
• la centratura è difficile

But in the plural, it changes to difficili:

• i compiti sono difficili
• le centrature sono difficili

So here, since centratura is singular, difficile stays singular too.

The sentence breaks down like this:

• La centratura = the centering / the alignment
• è = is
• più difficile = more difficult
• di quanto credevo = than I thought

So the structure is:

[subject] + [to be] + [comparative adjective] + [comparison clause]

That gives:

La centratura è più difficile di quanto credevo.

A very useful model to reuse is:

• X è più Y di quanto credevo
• X is more Y than I thought

Examples:

• Il test è più facile di quanto credevo.
  = The test is easier than I thought.
• Il lavoro è più lungo di quanto pensavo.
  = The job is longer than I thought.