Learn Spanish (Latin America): Él observó la patada, la cual fue perfecta, ya que quiere aprender. - He observed the kick, which was perfect, since he wants to learn.

In non-defining relative clauses (clauses surrounded by commas that add extra information), you can use either que or el/la cual. However, using la cual sounds slightly more formal and precise. It must agree in gender and number with the noun it refers to, which is the feminine singular la patada.

Why is it fue perfecta in the preterite instead of era perfecta in the imperfect?

Using fue treats the kick as a completed, momentary event that had a clear beginning and end. If we used era, it would sound like a background description of an ongoing characteristic, but a kick is a sudden, instantaneous action.

What does ya que mean here, and could I just use porque?

ya que translates to "since" or "given that". You could absolutely use porque (because) here, but ya que is a great transition phrase to know at this level to vary your sentence structure and sound more natural.

Why does the sentence switch to the present tense with quiere aprender when the rest is in the past?

The tenses reflect reality: he observed the kick in the past (observó), but his desire to learn is an ongoing fact that is still true right now in the present (quiere). If his desire only existed in the past, it would be quería aprender.