On pije previše kave, a ona pije previše čaja.

Because previše (too much) is a quantity word that requires the noun after it to be in the genitive case (partitive use). For mass nouns, that’s the genitive singular:

• kava → genitive singular kave
• čaj → genitive singular čaja

The same happens after other quantity words like puno, mnogo, dosta, malo:

• puno vode (a lot of water)
• malo vremena (a little time)

Both are grammatical but the focus differs:

• On pije previše kave. Emphasis on the amount of coffee being too large. Here previše quantifies the noun, so the noun is genitive: kave.
• On previše pije kavu. Emphasis on the action/frequency: he drinks coffee too much/too often. Here previše modifies the verb pije, so the direct object is in the accusative: kavu.

In everyday speech, the first version is the go‑to way to say he consumes too much coffee. Use the second when you want to stress the excessive habit rather than the amount.

• a ≈ “while/whereas.” It links two clauses with a mild contrast or a side‑by‑side comparison: He does X, whereas she does Y.
• i = “and.” Simple addition with no contrast.
• ali = “but.” A stronger, adversative contrast.

So On pije previše kave, a ona pije previše čaja presents a contrast in habits without strongly opposing them.

Croatian often drops subject pronouns, but here pije is the same form for “he,” “she,” and “it,” so the pronouns help avoid ambiguity. You can omit them if the context already makes clear who is being talked about:

• Context clear: Pije previše kave, a ona previše čaja.
• Out of the blue, keeping on/ona is clearer.

• ja pijem
• ti piješ
• on/ona/ono pije
• mi pijemo
• vi pijete
• oni/one/ona piju

Note the stem pij- in most forms.

Yes, gender shows up in the past participle:

• Masculine: On je pio previše kave.
• Feminine: Ona je pila previše čaja.

• č = “ch” in “chop”
• š = “sh” in “ship”
• j = “y” in “yes”
• ije = roughly “ee‑ye”

Approximation: “On PEE-yeh pre-VEE-sheh KAH-veh, a OH-nah PEE-yeh pre-VEE-sheh CHAH-yah.”

They can be both:

• As mass nouns (substance): use genitive singular after previše.
  • previše kave (too much coffee)
  • previše čaja (too much tea)
• As countable servings (cups): use genitive plural after previše.
  • previše kava (too many coffees)
  • previše čajeva (too many teas)

Counting examples:

• “two coffees” = dvije kave
• “two teas” = dva čaja

Yes, Croatian allows flexible word order to highlight what you want to emphasize:

• Neutral: On pije previše kave, a ona pije previše čaja.
• Emphasis on quantity: On previše kave pije, a ona previše čaja.
• Strong focus: Previše kave on pije, a ona previše čaja.

The meaning stays the same; the emphasis shifts.

The main noun difference is the word for coffee:

• Croatian: kava → genitive kave (as in the sentence)
• Serbian/Bosnian: kafa → genitive kafe: On pije previše kafe, a ona pije previše čaja.

The rest (verb pije, previše, čaja, conjunction a) is the same.

• pije = “drinks” (from piti)
• piše = “writes” (from pisati)

They look similar but mean very different things.

The adjective must agree with the genitive case required by previše:

• previše jake kave (too much strong coffee; fem. gen. sg.)
• previše vrućeg čaja (too much hot tea; masc. gen. sg.)

Use piti for general/habitual actions (as in the sentence). Popiti is perfective, focusing on a completed event:

• Habitual: On pije previše kave.
• One occasion (past): On je popio previše kave.
• Tendency with perfective: Zna popiti previše kave. (He tends to end up drinking too much coffee.)