L’acqua che cade dalla fontana crea un suono rilassante.

Che is the most common relative pronoun in Italian and serves as both subject and direct object. In this sentence it refers to l’acqua (the water) and functions as the subject of the subordinate verb cade.

• Cui requires a preposition (e.g. di cui, in cui).
• Il quale (or la quale) is more formal and must agree in gender/number, but che is by far the typical choice.

Cade is the third person singular present indicative of cadere (“to fall”). We use it because:

• The subject l’acqua is singular (“the water”).
• The present tense describes a general or ongoing action (“water falls”).

Crea is the third person singular present indicative of creare (“to create/produce”). Alternatives include:

• Produce (from produrre), equally correct.
• Fa (from fare), more colloquial but common: fa un suono rilassante.

In Italian, countable singular nouns normally require an article:

• Un is the indefinite article used here because we’re not specifying a particular sound.
• Omitting the article (suono rilassante) sounds ungrammatical.
• Using the definite article il would imply a specific, known sound.

Rilassante comes from the present participle of rilassare (“to relax”) and functions as an adjective meaning “relaxing.” Adjectives in -ante have:

• The same form for masculine and feminine in the singular (rilassante),
• A plural form in -i (rilassanti).

Yes.

• Cadere (“to fall”) emphasizes the action of falling.
• Scendere (“to descend/go down”) emphasizes movement along a path.
  Both are correct: L’acqua che scende dalla fontana also means “the water descending from the fountain.”