O desnível é grande.

Because desnível is being used as a specific noun, so Portuguese normally uses the definite article o here: O desnível.

Also, desnível is a masculine singular noun, so the article must be o.

In Portuguese, using the article with nouns is often more common than in English. So where English might sometimes say something more general without the, Portuguese often still uses o or a.

You usually learn a noun’s gender together with the noun itself: o desnível.

There is no completely reliable rule from the spelling alone, but the article tells you clearly here. Since it is o desnível, any adjective referring to it must agree as masculine singular too. In this sentence, grande has the same form for masculine and feminine singular, so you do not see a visible change.

Portuguese uses ser and estar differently, and here é comes from ser.

É is used because the sentence describes a characteristic of the desnível itself: it is large/significant. That is treated as an inherent or defining quality, not a temporary state.

So:

• O desnível é grande. = the height difference / unevenness is large.
• Está grande would sound much less natural here.

With terrain, construction, roads, steps, and physical features, ser is normally the right choice.

Because grande is the predicate adjective in the sentence.

The structure is:

O desnível + é + grande
subject + verb + adjective

This is completely normal in Portuguese, just like:

• A estrada é longa.
• A casa é pequena.

So grande is not directly sitting before the noun; it comes after the linking verb é.

Yes, but that would be a different structure.

• O desnível é grande. = full sentence
• um grande desnível = noun phrase, not a full sentence by itself

For example:

• Há um grande desnível na estrada.

In that case, grande comes before the noun because it is directly modifying it inside the noun phrase. In your original sentence, grande comes after é because it is the complement of the verb.

It is commonly used for a difference in level or height between two points.

In Portugal, you might hear it for things like:

• roads
• pavements
• floors
• steps
• land or terrain
• construction areas

So it often refers to a physical level difference, not just something abstract.

Yes, grande is natural, but in this kind of sentence it often means something closer to:

• large
• significant
• considerable

So it is not necessarily about physical size in a broad sense. It is more specifically saying the level difference is substantial.

In European Portuguese, the stress is on ní:

des-NÍ-vel

A rough approximation for an English speaker is:

duzh-NEE-vel

A more precise IPA version is:

/dɨʒˈnivɛɫ/

A few useful points:

• the s in des- sounds like zh here because it comes before a voiced consonant
• the stress falls on ní
• the final l is a dark l, as in European Portuguese generally

Yes, it is natural, especially in contexts involving terrain, roads, pavements, stairs, building work, or any place where a level difference matters.

It sounds like something you might read on a warning sign, hear in a practical description, or use when commenting on a physical space.

Yes. The most straightforward way is:

O desnível é muito grande.

That is the normal choice.

Other possibilities exist, depending on tone:

• O desnível é bastante grande. = quite large
• O desnível é enorme. = enormous
• Há um grande desnível. = there is a large height difference

The plural is:

Os desníveis são grandes.

Changes:

• o → os
• desnível → desníveis
• é → são
• grande → grandes

So the sentence becomes plural all the way through.

In a normal sentence like this, omitting it would sound wrong:

• O desnível é grande. = correct
• Desnível é grande. = not natural in standard Portuguese

Portuguese generally needs an article here unless you are using the noun in a very special context, such as a heading, label, or dictionary-style entry.