Learn Hardcore Greek: Πρέπει να αλλάξουμε την τροφή της γάτας, γιατί δεν τρώει καλά. - We have to change the cat’s food, because she isn’t eating well.

Πρέπει να αλλάξουμε… = We need to change…
So να is the marker that introduces that verb form, similar in meaning to English to in need to, but grammatically it works differently.

What form is αλλάξουμε and why isn’t it αλλάζουμε?

Αλλάξουμε is aorist subjunctive, 1st person plural, from αλλάζω / αλλάξω (to change).
After πρέπει να, Greek often chooses between:

present subjunctive: να αλλάζουμε = to be changing / to change repeatedly / as a general habit
aorist subjunctive: να αλλάξουμε = to change (once), to make a specific change Here, changing the cat’s food is a single decision, so να αλλάξουμε fits well.

Why does την τροφή use την? How do I know it’s accusative?

Because την τροφή is the direct object of αλλάξουμε (change what? → the food). Direct objects in Greek are typically in the accusative.
την is the feminine singular accusative article (the) used with τροφή (feminine):

nominative: η τροφή
accusative: την τροφή

What’s the difference between τροφή and φαγητό?

Both can mean food, but they’re used differently:

τροφή = food/nourishment/diet, often a bit more “nutrition/diet” sounding (also used for animal feed and diet changes).
φαγητό = food/a meal, more everyday and “what you eat for lunch/dinner.” For a cat’s diet, τροφή is very natural.

Why is it της γάτας and not την γάτα?

της γάτας is genitive and shows possession/association:

την τροφή της γάτας = the cat’s food / the food of the cat
If you said την γάτα, that would be accusative and would mean the cat is the direct object (e.g., feed the cat, change the cat—not what you want).

What exactly does the genitive do in της γάτας?

Here it functions like English ’s:

η τροφή της γάτας = the cat’s food
Greek commonly uses article + noun in the genitive to express ownership: της γάτας (of the cat).

Why is γιατί used here—doesn’t it mean why?

γιατί can mean both why (a question) and because (an explanation).
In this sentence, it clearly means because:

…, γιατί δεν τρώει καλά. = …, because she/he isn’t eating well.
If it were a question, you’d usually see a question mark and different intonation: Γιατί δεν τρώει καλά; = Why isn’t she/he eating well?

How does negation work in δεν τρώει?

For most verbs in the present/past indicative, you negate with δεν:

τρώει = (she/he) eats / is eating
δεν τρώει = (she/he) doesn’t eat / isn’t eating With να-clauses (subjunctive), you usually use μη(ν) instead, but here it’s not a να-clause, so δεν is correct.

Is τρώει present tense? Does it mean eats or is eating?

Yes, τρώει is present tense, 3rd person singular of τρώω (to eat).
Greek present can correspond to either:

eats (habitually), or
is eating (right now) Context decides. Here it’s more like a general current situation: she/he isn’t eating well.

What does καλά add—does it mean “good” or “well”?

καλά is an adverb meaning well (or properly / much / satisfactorily, depending on context).
So δεν τρώει καλά means the cat isn’t eating well—maybe not enough, not enthusiastically, or not properly.

Could I replace γιατί with επειδή?

Often yes:

γιατί = very common, neutral, also used in speech a lot.
επειδή = also because, sometimes slightly more “explicit/structured,” common in writing too. Both work here with essentially the same meaning.

What’s a natural English-like word order for this sentence, and can Greek word order change?

A very close mapping is:

Πρέπει να αλλάξουμε την τροφή της γάτας = We need to change the cat’s food
γιατί δεν τρώει καλά = because she/he isn’t eating well
Greek word order is more flexible than English because case endings/articles show roles. You could move parts for emphasis (especially the reason clause), but this order is already very natural.