Notei que o índice do livro era curto e muito claro.

Notei is the 1st person singular of the pretérito perfeito of notar (to notice).

• eu notei = I noticed
• It refers to a completed action in the past.

So the speaker is saying that, at some moment, they noticed something.

Compare:

• Notei que... = I noticed that...
• Noto que... = I notice that...
• Notava que... = I used to notice / I was noticing that... (less likely here)

Here, que means that and introduces a subordinate clause:

• Notei que o índice do livro era curto e muito claro.
• I noticed that the book’s index was short and very clear.

In Portuguese, this que is very common after verbs such as:

• achar que = to think that
• ver que = to see that
• dizer que = to say that
• notar que = to notice that

Unlike English, where that is often dropped, Portuguese usually keeps que.

Portuguese uses the definite article much more often than English.

So o índice literally means the index, and in Portuguese that article often sounds natural even where English might omit it in some contexts.

Here it is referring to a specific index:

• o índice do livro = the index of the book / the book’s index

This is very normal in European Portuguese.

Do is a contraction:

• de + o = do

So:

• o índice do livro = the index of the book

Other common contractions:

• de + a = da
• de + os = dos
• de + as = das

This kind of contraction is extremely common in Portuguese and is required in standard usage.

Era is the imperfect form of ser, while foi is the preterite.

Here, era is used because the sentence is describing a state or characteristic of the index, not a single completed event.

• era curto e muito claro = it was short and very clear

This feels like a description the speaker is making about how the index was, from their point of view at that time.

Very roughly:

• foi = it was (as a completed fact/event)
• era = it was (as a description/background/ongoing state)

In this sentence, era sounds more natural because short and clear are descriptive qualities.

Usually, era is the better choice here.

Why?

• ser is used for more inherent or defining characteristics
• estar is used for more temporary states or conditions

So:

• o índice era curto e muito claro = the index was short and very clear
  This sounds like a description of its nature/content.

If you said:

• o índice estava claro

that could sound more like it was clear at that moment or in a more situational sense. For example, maybe a scan or display was clear.

In this sentence, era is the natural choice because the speaker is judging the index’s qualities.

Because they agree with índice, which is a masculine singular noun.

• o índice → masculine singular
• therefore: curto and claro must also be masculine singular

If the noun were feminine singular, the adjectives would normally change:

• a secção era curta e clara

Agreement is a key feature of Portuguese grammar:

• noun and adjective usually match in gender and number

Here curto means something like short / brief / not long.

For a text-related thing like an index, it suggests that the index did not have many entries or was not extensive.

A native English speaker might wonder if pequeno could be used instead. Sometimes yes, but the nuance changes:

• curto focuses on length/extent
• pequeno focuses more on size/smallness

So curto makes good sense if the speaker is thinking about the index’s length or amount of content.

Not here.

In this sentence, claro means clear, in the sense of:

• easy to understand
• well organized
• not confusing

So muito claro means very clear.

Portuguese claro can mean different things depending on context:

• uma cor clara = a light color
• uma explicação clara = a clear explanation
• um índice claro = a clear, easy-to-use index

Because here muito is an adverb, not an adjective.

It modifies claro:

• muito claro = very clear

Adverbs do not agree in gender or number.

Compare:

• muito claro = very clear
  (muito = adverb)

but:

• muitos livros = many books
  (muitos = adjective/determiner, so it agrees)

This is a very common point for learners because muito can behave in more than one way.

Yes, índice can mean index in a book. Depending on context, it can also refer to a table of contents or another kind of index/listing.

That means the exact nuance can depend on context:

• índice may refer to the list of contents/sections
• in some contexts, a more specific term like índice remissivo may be used for an alphabetical back-of-the-book index

But for a learner, the important point is that índice is a normal and correct word here.

Yes, it could be reversed:

• curto e muito claro
• muito claro e curto

Both are grammatically possible.

The current order feels natural because the speaker first mentions one quality (short) and then adds another, slightly more evaluative one (very clear).

Portuguese is fairly flexible with this kind of adjective phrase after a verb like ser, but the original order sounds completely normal.

Yes, but the tone changes.

In European Portuguese, aperceber-se de que is also common for to realize / notice that:

• Apercebi-me de que o índice do livro era curto e muito claro.

That sounds a little more like I realized than simple I noticed.

Compare:

• notei que = I noticed that
• apercebi-me de que = I realized / became aware that

Also notice the structure:

• aperceber-se de que
• not just aperceber que

So notei que is simpler and very natural, while apercebi-me de que is also correct but slightly different in feel.