Mi primo dejó de tomar refrescos porque cada vaso tenía demasiadas calorías.

Dejar de + infinitive means to stop (doing something), to quit.

• Dejó de tomar = he stopped drinking / he quit drinking.
• If you say dejó tomar, it means he allowed (someone) to drink, because dejar + infinitive without de usually means to allow/let.
• You can say paró de tomar, and people will understand, but dejar de + infinitive is much more common and natural for “quit a habit.”

In Latin America, tomar is very commonly used with drinks and is often more frequent than beber in everyday speech.

• tomar refrescos = to drink soft drinks
• beber refrescos = also correct, just a bit more neutral/formal in many places.
  Tomar can also mean to take (e.g. tomar un taxi, tomar medicina), so context tells you it means “drink” here.

Spanish often uses the plural when talking about a type of drink in general, especially in the sense of a repeated habit.

• dejó de tomar refrescos = he quit drinking sodas in general / soft drinks as a category.
  You could say dejó de tomar refresco, but it sounds more like “a soft drink” in a particular situation, not the whole category. The plural is more natural for “stopped drinking soda” as a habit.

After dejar de + infinitive, when talking about a general activity or habit, Spanish often omits the article:

• dejó de fumar, dejó de comer carne, dejó de tomar refrescos.
  No article here emphasizes the activity in general (drinking soda), not specific sodas. Dejó los refrescos is also possible but changes the nuance (see next question).

Cada is always followed by a singular noun in Spanish.

• cada vaso, cada persona, cada día (never cadas and never plural after it).
  Even though logically you’re talking about many glasses, grammatically it’s cada + singular: “each glass had…” → cada vaso tenía.

Dejó (preterite) refers to a specific completed action in the past: there was a particular moment when he stopped.

• Mi primo dejó de tomar refrescos = at some point, he quit.
  Dejaba de tomar refrescos would suggest something like “he was in the process of stopping / he used to stop,” which doesn’t match the idea of a single, clear decision as well.

Demasiadas is an adjective that must agree in gender and number with the noun it modifies.

• calorías is feminine plural → demasiadas calorías.
  Use:
• demasiado (masc. sing.) → demasiado azúcar
• demasiada (fem. sing.) → demasiada sal
• demasiados (masc. pl.) → demasiados refrescos
• demasiadas (fem. pl.) → demasiadas calorías

In Spanish, gender is mostly arbitrary and must be memorized with each noun.

• la caloría / las calorías → feminine.
  There are some patterns (e.g. many nouns ending in -a are feminine), but there are many exceptions. The safest approach is to always learn the noun with its article: la caloría.

Because it’s modified by demasiadas, which already gives a quantity idea. With quantity words, Spanish commonly drops the article:

• muchas calorías, pocas calorías, demasiadas calorías, tantas calorías.
  Adding an article (las demasiadas calorías) would sound marked or different in meaning; here the natural structure is just demasiadas calorías.

Yes, but it depends on the country. Different regions use different words for “soda/soft drink”:

• refresco(s) – widely understood, common in many countries.
• gaseosa – very common in places like Argentina, Peru, Colombia, etc.
• soda – used in some countries/contexts.
  Tomar gaseosa or beber gaseosa is natural where gaseosa is the usual word, but tomar refrescos is safely understood across Latin America.

By itself, dejó de only tells you that he stopped at some point in the past, not whether it was permanent.
Context would clarify:

• Mi primo dejó de tomar refrescos para siempre → clearly forever.
• Mi primo dejó de tomar refrescos por un tiempo → just for a while.
  So the sentence alone doesn’t guarantee it’s permanent, only that he quit at that point.

The full sentence with porque… simply adds the reason; the basic structure and word order are neutral and natural:

• Main clause: Mi primo dejó de tomar refrescos
• Reason clause: porque cada vaso tenía demasiadas calorías
  You could move the reason to the front (Porque cada vaso tenía demasiadas calorías, mi primo dejó de tomar refrescos) for stylistic emphasis on the cause, but the meaning remains the same.