O nível diminuiu.

O is the masculine singular definite article, meaning the.

In O nível diminuiu, it tells you that you are talking about the level, not just a level.

• o nível = the level
• um nível = a level

Portuguese usually uses articles more often than English, so leaving it out can sound incomplete in many contexts.

Because nível is a masculine noun in Portuguese, so it takes o in the singular:

• o nível = the level
• os níveis = the levels

This is something you mostly just have to learn with the noun. The ending -el does not automatically guarantee gender in every case, but nível is masculine.

Nível is a noun. It means level in many of the same senses as in English, such as:

• water level
• energy level
• difficulty level
• noise level
• blood sugar level

So this sentence can be used in many different contexts depending on what level is being discussed.

In Brazilian Portuguese, nível is pronounced roughly like NEE-vehw or NEE-vew, with stress on the first syllable:

• ní- = stressed
• -vel = unstressed

A few helpful points:

• The accent mark in nível shows the stress goes on ní.
• The e in the second syllable is usually a clear short vowel.
• The final l in Brazilian Portuguese often sounds like a w-like sound.

So it sounds approximately like NEE-vew.

Diminuiu comes from the verb diminuir, which means to decrease, to diminish, or to go down depending on context.

So:

• diminuir = infinitive, to decrease
• diminuiu = decreased / went down

It is the form used for ele/ela/você in the simple past.

It is the pretérito perfeito in Portuguese, often called the simple past.

Here it means that the decrease happened as a completed event:

• O nível diminuiu. = The level decreased / went down.

This tense is commonly used for something that happened and is viewed as complete.

Because diminuir is an -ir verb, and in the pretérito perfeito, the ele/ela/você form ends in -iu.

Compare:

• subir → subiu
• abrir → abriu
• diminuir → diminuiu

With diminuir, you get diminu- + iu, which creates the sequence uiu in writing: diminuiu.

It may look unusual at first, but it is a normal spelling.

Because the verb stem already contains an i: diminuir.

When you conjugate it in this tense, the ending adds another i:

• diminuir
• stem: diminu-
• ending: -iu
• result: diminuiu

So the spelling reflects how the verb is built. This is normal in Portuguese and not a typo.

In Brazilian Portuguese, it is roughly pronounced jee-mee-noo-EE-ooh or djee-mee-noo-EE-u, with the stress on the last iu part.

A helpful breakdown:

• di often sounds like djee in many Brazilian accents
• mi = mee
• nu = noo
• iu = a quick ee-ooh glide

So the whole word is approximately djee-mee-noo-EE-ooh.

Yes, depending on context.

Portuguese pretérito perfeito often corresponds to English simple past, but in some contexts English may naturally use present perfect:

• O nível diminuiu. = The level decreased.
• also possible in context: The level has decreased.

English chooses between those options based on context and style, but the Portuguese sentence itself is completely normal.

Yes, in many contexts you can, but the nuance can be slightly different.

• diminuiu focuses on becoming smaller/lower
• abaixou focuses more directly on going down

For something like water level, temperature, or a measured amount, both may work depending on context.
Diminuiu is often a bit more neutral and widely usable for quantities or levels in general.

Yes. Nível is used very broadly, just like level in English.

Examples:

• O nível de água diminuiu. = the water level decreased
• O nível de estresse diminuiu. = the stress level decreased
• O nível de dificuldade diminuiu. = the difficulty level decreased

So the sentence can be physical, numerical, emotional, technical, and more.

Because the subject is singular: o nível.

In Portuguese, the verb agrees with the subject:

• O nível diminuiu. = singular
• Os níveis diminuíram. = plural

So if you change nível to níveis, the verb must also change.

You put não before the verb:

• O nível não diminuiu. = The level did not decrease.

This is the standard way to negate a sentence in Portuguese.

In Brazilian Portuguese, you can often keep the same word order and use intonation:

• O nível diminuiu? = Did the level decrease?

In writing, the question mark shows it clearly. Portuguese usually does not need a helper verb like did.

Usually, no—not in a normal full sentence like this.

O nível diminuiu sounds natural because Portuguese normally wants the article with this noun.
Without the article, Nível diminuiu sounds unnatural or incomplete in most everyday contexts.

You might see article omission in headlines, notes, or very specialized styles, but for normal speech, use o nível.