Ieri pensavo che il fusibile fosse rotto, invece era soltanto la lampadina.

Because ieri just tells you when the thought happened; it does not automatically require the passato prossimo.

Here, pensavo describes an ongoing mental state in the past: I was thinking / I thought. The imperfect is very common for background thoughts, beliefs, intentions, and states of mind.

So:

• Ieri pensavo che... = Yesterday I was thinking / I thought that...
• Ieri ho pensato che... would usually sound more like Yesterday it occurred to me that... or I had the thought that..., often as a more completed, single event.

In this sentence, the imperfect fits well because the speaker is describing a mistaken belief they had at that time.

Because after verbs of thinking, believing, doubting, fearing, etc., Italian often uses the subjunctive in a subordinate clause introduced by che.

So:

• pensavo che... often leads to the subjunctive
• fosse is the imperfetto del congiuntivo of essere

That is why you get:

• pensavo che il fusibile fosse rotto

and not normally:

• pensavo che il fusibile era rotto

A useful pattern is:

• present main verb -> present/past subjunctive
• past main verb -> imperfect/pluperfect subjunctive

Here the main verb is past (pensavo), so the subordinate clause naturally uses the imperfect subjunctive: fosse.

Fosse is the imperfect subjunctive form of essere for both singular and plural persons in certain contexts.

In this sentence, it means was in the sense required after pensavo che.

Some forms of the imperfect subjunctive of essere are:

• io fossi
• tu fossi
• lui/lei fosse
• noi fossimo
• voi foste
• loro fossero

So il fusibile fosse rotto means the fuse was broken, but expressed inside a clause that requires the subjunctive.

Because rotto agrees with il fusibile, which is a masculine singular noun.

Agreement works like this:

• il fusibile rotto = masculine singular
• la lampadina rotta = feminine singular
• i fusibili rotti = masculine plural
• le lampadine rotte = feminine plural

Since the thing being described in that clause is il fusibile, the adjective/past participle must be rotto.

Che introduces the subordinate clause: the content of the thought.

So:

• pensavo = I thought / I was thinking
• che il fusibile fosse rotto = that the fuse was broken

It works very much like English that, although in Italian che is often more necessary and natural than that is in English.

Invece means instead, whereas, or but actually, depending on context.

Here it marks a correction or contrast:

• first idea: pensavo che il fusibile fosse rotto
• reality: invece era soltanto la lampadina

So it signals: that was not the real problem; the real problem was something else.

It is a very common discourse word for correcting expectations or introducing a contrast.

Because Italian often leaves out information that is already clear from context.

The full idea is something like:

• invece era soltanto la lampadina
  = instead, it was only the light bulb
• understood meaning: the only thing that was broken was the light bulb or it was just the light bulb that was the problem

The adjective rotta is not repeated because it is easy to infer from the previous clause. This kind of omission is very natural in Italian.

If you wanted to make it fully explicit, you could say:

• invece era rotta soltanto la lampadina
• invece era soltanto la lampadina ad essere rotta

But the original version sounds lighter and more idiomatic.

Because the second clause is not dependent on pensavo che. It is a new main clause stating what was actually true.

Compare the two parts:

• pensavo che il fusibile fosse rotto
  subordinate clause after pensavo -> subjunctive
• invece era soltanto la lampadina
  independent statement of fact -> indicative

So fosse expresses the speaker's earlier belief, while era gives the real situation.

Yes. In this sentence, solo, soltanto, and solamente are all possible.

So these are all acceptable:

• era soltanto la lampadina
• era solo la lampadina
• era solamente la lampadina

In everyday speech, solo is probably the most common. Soltanto can sound a little more careful or emphatic, but the difference is small here.

Italian uses definite articles much more often than English does.

When talking about a specific thing in the situation, Italian normally says:

• il fusibile
• la lampadina

Even if English might sometimes say the fuse and the bulb, or in some contexts just fuse and bulb, Italian strongly prefers the article.

Without the article, the sentence would sound unnatural here.

Yes. Il fusibile is the standard word for an electrical fuse.

Depending on the exact context, native speakers may also use other expressions for electrical problems, for example:

• è saltato il fusibile = the fuse blew
• il fusibile è bruciato = the fuse is blown/burnt out

So rotto is understandable, but with a fuse, Italians often prefer saltato or bruciato if they want to sound especially natural.

Yes, that is possible, but it changes the structure slightly.

Compare:

• pensavo che il fusibile fosse rotto
  = I thought the fuse was broken
• pensavo che si fosse rotto il fusibile
  = I thought the fuse had broken / blown

The second version uses the verb rompersi rather than essere + rotto. Both can work, but the nuance is slightly different:

• fosse rotto describes the fuse's state
• si fosse rotto focuses more on the event of breaking

For a fuse specifically, many speakers would more naturally say:

• pensavo che fosse saltato il fusibile

The basic order is quite normal:

• Ieri = time expression
• pensavo = main verb
• che il fusibile fosse rotto = subordinate clause
• invece = contrast marker
• era soltanto la lampadina = main clause giving the correction

Italian word order is somewhat flexible, but the original order sounds natural and clear.

For example, you could also hear:

• Pensavo ieri che il fusibile fosse rotto...
• Invece era solo la lampadina.

But Ieri pensavo che il fusibile fosse rotto, invece era soltanto la lampadina is a very straightforward, natural way to say it.

In standard Italian, pensavo che normally takes the subjunctive, especially in careful written language:

• pensavo che fosse...

In informal speech, some speakers may occasionally use the indicative, especially in regional or colloquial usage, but for learners the safest and most standard choice is:

• pensavo che + congiuntivo

So this sentence is a good example to remember:

• pensavo che il fusibile fosse rotto