Credevo che il link fosse corretto, ma la schermata mostrava un errore di accesso.

Credevo is the imperfetto of credere. Here it suggests a state of mind in the past: I thought / I believed... as background information.

Italian often uses the imperfetto for:

• ongoing past states
• mental attitudes
• background description

So Credevo che... sounds very natural for I thought that...

If you said ho creduto, it would usually sound more like:

• I believed it (for a certain moment / until something happened)

That tense is possible in some contexts, but here credevo fits much better because it introduces what the speaker believed before discovering the problem.

Because after credere che in this kind of sentence, Italian normally uses the subjunctive.

So:

• Credevo che il link fosse corretto = I thought the link was correct

Here, fosse is the imperfetto congiuntivo of essere.

A native English speaker often expects era because English says I thought the link was correct, but Italian grammar treats this as a belief/opinion, so the subordinate clause takes the subjunctive.

In everyday informal speech, you may sometimes hear era, but fosse is the standard and best choice.

This is due to the sequence of tenses.

The main verb is in the past:

• Credevo = I thought / I used to think

Because the main verb is in a past tense, the dependent subjunctive usually also shifts into a past form:

• che... fosse

So:

• Credo che sia corretto = I think it is correct
• Credevo che fosse corretto = I thought it was correct

That is a very common pattern in Italian.

Mostrava is also imperfetto, and it gives a descriptive, background feel:

• the screen was showing an access error
• the screen showed an access error

It presents the situation as part of the scene the speaker was facing.

If you said ha mostrato, it would sound more like a single completed event:

• the screen displayed an error (at one particular moment)

Both can be possible depending on context, but mostrava works well if you are narrating what the screen said/displayed at that time.

Schermata usually means:

• screen
• screen page
• screen display
• interface screen

In tech contexts, it often refers to what appears on the device at a particular moment, not just the physical monitor.

So:

• la schermata mostrava... = the screen/the page displayed...

This is often more natural than schermo, which is more literally the physical screen.

Literally, it means access error.

In practice, it could refer to things like:

• a login problem
• permission denied
• inability to open a page/resource
• an authentication/access issue

The phrase di accesso means related to access.

So:

• un errore di accesso = an access error / an access-related error

Depending on the context, English might translate it more naturally as:

• access error
• login error
• access denied error

Italian uses articles more often than English.

So it is normal to say:

• il link
• la schermata
• un errore

Even where English might say:

• I thought the link was correct, but screen showed access error
  Italian generally needs the articles:
• la schermata
• un errore

In this sentence:

• il link = the specific link being discussed
• la schermata = the screen/page being looked at
• un errore = an error message, not previously specified

Yes, link is normally treated as masculine in Italian:

• il link
• i link

It is also usually invariable, meaning the singular and plural forms are the same:

• un link
• due link

This is common with many borrowed English tech words in Italian.

Yes. Italian often omits subject pronouns when they are clear from the verb ending.

So:

• (Io) credevo = I thought

The -evo ending tells you it is I.

Likewise, in the second clause the subject is explicit:

• la schermata mostrava...

So the sentence does not need io unless you want emphasis:

• Io credevo che il link fosse corretto... This would sound more contrastive or emphatic.

Yes, very often.

• Pensavo che il link fosse corretto = I thought the link was correct
• Credevo che il link fosse corretto = I believed/thought the link was correct

The difference is subtle:

• pensavo often sounds like I thought
• credevo can sound slightly closer to I believed / I was under the impression

In many everyday contexts, both are possible.

Yes, but corretto is the more natural choice here.

• corretto = correct, valid, accurate
• giusto = right, correct, fair, appropriate

For a link, corretto sounds better because you are talking about technical correctness or validity.

So:

• il link fosse corretto = the link was correct/valid

Giusto is not impossible, but it is less precise in this context.

Because ma means but, and it connects two full clauses:

• Credevo che il link fosse corretto
• ma la schermata mostrava un errore di accesso

In Italian, it is very normal to put a comma before ma when it joins two independent clauses. It helps separate the contrast clearly.

Yes, exactly.

In tech Italian, mostrare un errore or mostrare un messaggio di errore is very common.

So:

• la schermata mostrava un errore di accesso

naturally means something like:

• the screen displayed an access error
• the page showed an access error message

Even though the wording is slightly abstract, it is a normal way to describe what appeared on screen.