Credevo che la ricotta sarebbe bastata, invece ho dovuto comprare un altro barattolo di yogurt.

Credevo is the imperfetto, which is very common for past thoughts, beliefs, and assumptions. It presents the belief as a background mental state: I thought / I believed.

Here, the speaker is describing what they believed before discovering that reality was different.

Ho creduto is possible in other contexts, but it usually sounds more like a completed act of believing, not the natural background idea found here.

Che means that and introduces the clause that explains what the speaker believed:

• Credevo che... = I thought that...

In English, that is often omitted, but in Italian che is normally required.

This is the form Italian uses to express a future-in-the-past idea.

From the past point of view of credevo, the speaker expected that later the ricotta would be enough. Italian expresses that with conditional past:

• sarebbe bastata

Even though the form is technically a past conditional, in English it often translates simply as would be enough, not necessarily would have been enough.

Because bastasse would not express the same time relationship.

• sarebbe bastata = the speaker thought that, later / in the end, the ricotta would be enough
• bastasse would give a different nuance, more like were enough or might be enough

So the sentence uses sarebbe bastata because it is looking forward from a past moment.

Because it agrees with la ricotta, which is feminine singular.

The verb bastare uses essere in compound tenses, so the past participle agrees with the subject:

• la ricotta sarebbe bastata
• il formaggio sarebbe bastato
• le uova sarebbero bastate

Bastare means to be enough or to suffice.

The important thing for English speakers is that the thing that is enough is the subject:

• La ricotta basta = The ricotta is enough
• La ricotta sarebbe bastata = The ricotta would have been / would be enough

If you want to say enough for someone, Italian often adds an indirect object:

• Mi basta = It’s enough for me
• Ci bastava = It was enough for us

Ho dovuto comprare presents the necessity as a specific past fact: the speaker found themselves in the situation of having to buy more yogurt.

Dovevo comprare would sound more ongoing, less definite, or sometimes like I was supposed to buy.

So here:

• ho dovuto comprare = a concrete necessity in the story
• dovevo comprare = more open-ended or descriptive

Because after modal verbs like dovere, potere, and volere, Italian normally uses the infinitive directly:

• ho dovuto comprare
• posso andare
• voglio mangiare

So comprare follows dovuto without any preposition.

Invece marks a contrast. Here it means something like but instead, however, or as it turned out.

The sentence is structured like this:

• expectation: Credevo che la ricotta sarebbe bastata
• reality: invece ho dovuto comprare un altro barattolo di yogurt

So invece is the word that signals: that expectation was wrong.

Because the speaker is talking about another container of yogurt, not just an unspecified extra amount.

• un altro barattolo di yogurt = another jar / tub of yogurt

Italian often uses container words very naturally in these situations. Depending on the product, you might also hear:

• vasetto = small pot
• confezione = package / pack

So barattolo makes the quantity more specific.

Because Italian often uses the definite article with food nouns when referring to a specific item or amount already understood from the context.

So la ricotta here means something like:

• the ricotta we had
• the ricotta in question

English often leaves the article out in these cases, but Italian usually sounds more natural with it.

Yes, absolutely.

Both are natural:

• Credevo che...
• Pensavo che...

The difference is small:

• pensavo is often a more neutral I thought
• credevo can sound a bit more like I believed / I assumed

In this sentence, either one works well.