Eu não sabia que mandioca com manteiga ficava tão boa.

Sabia is the imperfect of saber, and it describes a state in the past: I didn’t know.

That fits this sentence because the speaker is talking about their lack of knowledge before discovering it.

Soube is different. It often means found out / learned / heard and focuses more on the moment of finding out.

• Eu não sabia... = I didn’t know...
• Eu soube que... = I found out that...

So Eu não sabia que... is the natural choice here.

Here que means that and introduces the clause after sabia:

• Eu não sabia que... = I didn’t know that...

In Portuguese, que is normally used in this structure. In English, that is sometimes omitted, but in Portuguese it is usually kept.

Portuguese can use a noun without an article when talking about something in a general way, especially with foods.

So:

• mandioca com manteiga = cassava with butter, in a general sense

This sounds natural in Brazilian Portuguese. You may also hear a mandioca com manteiga, but without the article it sounds a little more like a general food combination rather than pointing to a specific serving.

Yes, it comes from ficar.

Ficar is a very flexible verb in Portuguese. It can mean:

• to stay
• to become
• to turn out
• to be / end up being in some contexts

In food-related sentences, ficar often means something like:

• to turn out
• to come out
• to taste
• to be in a resulting way

So mandioca com manteiga ficava tão boa is roughly like saying:

• cassava with butter turned out so good
• cassava with butter was so good
• cassava with butter tasted so good

This is about nuance.

Ficava is imperfect, and here it sounds more like a general quality or the speaker’s past realization about how that combination turns out.

• ficava tão boa = was so good / turned out so good / came out so good

If you said ficou tão boa, it would sound more tied to one specific occasion or one specific serving:

• A mandioca com manteiga ficou tão boa.
  = The cassava with butter turned out so good.

So ficava is a better fit if the idea is more like: I didn’t know cassava with butter was that good.

Yes, but the nuance changes.

• era tão boa: sounds more like a general quality
• estava tão boa: sounds more like a specific serving at that moment
• ficava tão boa: suggests how it turns out / how it is in that preparation

So:

• mandioca com manteiga era tão boa = cassava with butter was so good
• mandioca com manteiga estava tão boa = the cassava with butter was so good
• mandioca com manteiga ficava tão boa = cassava with butter turned out / came out so good

In this sentence, ficava is very natural because it highlights the result of the combination.

Because boa agrees with mandioca, and mandioca is feminine in Portuguese.

• a mandioca
• therefore: boa

Even though the sentence also has manteiga, the adjective is describing mandioca com manteiga as the main thing, and the head noun is still mandioca, so the adjective stays feminine singular.

Tão means so.

• tão boa = so good

It is used to intensify the adjective boa.

Other examples:

• tão bonito = so beautiful
• tão caro = so expensive
• tão gostoso = so tasty

Yes. In Brazilian Portuguese, Eu is often optional because sabia already shows the subject is I.

So both are natural:

• Eu não sabia que mandioca com manteiga ficava tão boa.
• Não sabia que mandioca com manteiga ficava tão boa.

Including Eu can add a little emphasis or clarity, but it is not required.

Usually com manteiga just means with butter, without being very specific.

Depending on context, it could mean:

• served with butter
• eaten with butter
• topped with butter
• prepared with some butter

If the speaker wanted to be more specific, they could say something else, such as:

• na manteiga = in butter / cooked with butter
• com manteiga por cima = with butter on top

But com manteiga is a natural general way to say it.

It can suggest both, but it most naturally sounds like a past realization about a general truth.

The speaker probably tried it at some point and realized:

• I didn’t know cassava with butter was so good.

So the discovery happened in one moment, but what was discovered is a more general fact about that food combination. That is one reason ficava works well here.