Eu quero escalar a montanha hoje.

In Portuguese, the verb ending often tells you who the subject is. Quero already means “I want,” so you can drop eu without losing meaning.
• With pronoun: Eu quero escalar a montanha hoje. (adds emphasis or clarity)
• Without pronoun: Quero escalar a montanha hoje. (very common in speech)

Querer means to want and is followed directly by an infinitive (no preposition needed). The structure is:
• querer (conjugated) + infinitive → quero escalar = I want to climb

You include a because you’re talking about a specific mountain (the definite article for feminine singular nouns).
• Specific mountain: escalar a montanha (climb the mountain)
• General idea: escalar montanhas (climb mountains) – no article
• One mountain, non-specific: escalar uma montanha (climb a mountain)

Both can mean to climb or go up, but:
• escalar often implies a technical climb or mountaineering.
• subir is more general: you “go up” something (stairs, a hill, a mountain).
So if you mean a challenging ascent, escalar a montanha is more precise.

Adverbs of time like hoje are flexible in Portuguese. You can say:
• Hoje eu quero escalar a montanha.
• Eu quero escalar a montanha hoje. (common in speech)
Position depends on emphasis and rhythm; both are correct.

No. In Portuguese, querer + infinitive is a direct construction. You do not use a preposition.
Correct: quero escalar
Incorrect: quero de escalar

To avoid repeating montanha, use the feminine singular direct-object pronoun a and attach it to the infinitive:
• Eu quero escalá-la hoje.
In European Portuguese, pronouns attach to the infinitive with a hyphen.

Nouns ending in -a are usually feminine in Portuguese. For montanha:
• Definite article: a montanha (singular), as montanhas (plural)
• Indefinite article: uma montanha, umas montanhas
• Direct-object pronoun: a (singular), as (plural)
Always match gender (feminine) and number (singular/plural).