A encosta é difícil de subir.

Because encosta is a feminine noun in Portuguese. That means it takes the feminine singular article a.

• a encosta = the slope / the hillside
• If it were a masculine noun, it would take o

This is something you simply learn with the noun: encosta is feminine.

É comes from ser, which is normally used for a more general or inherent characteristic.

So A encosta é difícil de subir means the slope is difficult to climb in general.

If you said A encosta está difícil de subir, that would sound more temporary or situational, as if current conditions are making it difficult right now, for example because it is muddy, slippery, or very hot.

So:

• é difícil = generally difficult
• está difícil = difficult at the moment / under current conditions

Some Portuguese adjectives have the same form for masculine and feminine singular. Difícil is one of them.

So you get:

• o caminho difícil
• a encosta difícil

The adjective does change in the plural:

• difíceis = difficult, plural

So the lack of change here is normal.

Here, de + infinitive is part of a very common Portuguese pattern:

adjective + de + infinitive

So:

• difícil de subir = difficult to climb
• fácil de entender = easy to understand
• impossível de fazer = impossible to do

In English, we usually use to + verb. In Portuguese, after many adjectives, the natural structure is de + infinitive.

Because when the adjective describes a noun and the infinitive explains what is difficult about that noun, Portuguese very often uses de + infinitive.

So:

• A encosta é difícil de subir = The slope is difficult to climb

This is similar to:

• O livro é fácil de ler = The book is easy to read

Without de, the structure would usually be different. For example:

• É difícil subir a encosta = It is difficult to climb the slope

That sentence is also correct, but it is built differently.

Yes, absolutely.

Both are correct, but the focus is slightly different:

• A encosta é difícil de subir
  Focuses on the slope as the thing being described.

• É difícil subir a encosta
  Focuses more on the action of climbing the slope.

In English, the difference is similar to:

• The slope is difficult to climb
• It is difficult to climb the slope

Both are natural.

No specific subject is needed here. The sentence is general, so the person doing the climbing is understood as anyone / people in general.

This is very common in both Portuguese and English:

• A encosta é difícil de subir
• The slope is difficult to climb

Neither sentence says exactly who is climbing. It is just understood in a general sense.

Yes. Encosta is understood as the object of subir, even though it appears earlier in the sentence.

This kind of structure is very common:

• O livro é fácil de ler
  The book is the thing you read.
• A cadeira é confortável de usar
  The chair is the thing you use.
• A encosta é difícil de subir
  The slope is the thing you climb.

So even though encosta is the grammatical subject of é, it is also the thing affected by subir in meaning.

It can mean both, depending on context.

In this sentence, subir could be understood as:

• to climb
• to go up
• sometimes even to ascend

With encosta, English often uses climb very naturally, but go up the slope is also possible.

So subir is a flexible verb whose best translation depends on the situation.

Normally, no. In this structure, de is the natural choice.

So you should say:

• difícil de subir

not:

• difícil para subir

After adjectives like difícil, fácil, and impossível, Portuguese commonly uses de + infinitive.

Yes, it is natural and idiomatic in European Portuguese.

It sounds like a normal way to describe terrain, for example in conversation, travel, hiking, or giving directions.

A European Portuguese speaker would readily understand it as a standard sentence meaning that the slope is hard to go up.