Learn Swedish: Vi ska se det höga berget idag. - We will see the high mountain today.

This is called double definiteness. When a definite noun (the mountain) has an adjective in front of it (high), Swedish requires the concept of "the" to appear twice: once as the separate article det and once as the noun ending -et in berget.

Why is the adjective höga instead of hög or högt?

When you use a definite article like det or den, the adjective has to be in its definite form. For almost all adjectives in Swedish, the definite form simply adds an -a to the end, changing hög to höga.

How do we know to use det instead of den here?

It depends on the noun's gender. The word for mountain is an ett-word (ett berg). Because of this, you must use the matching ett-word article det. If it were an en-word, you would use den.

Why is the verb se and not the present tense ser?

Se is the base dictionary form (infinitive) of the verb. We use it here because it comes right after the helper verb ska (will). Just like in English, you say "will see," not "will sees."