Ο σολομός είναι ακριβός, γι’ αυτό συνήθως τρώω βρώμη το πρωί και ψάρι μόνο το Σαββατοκύριακο.

Greek very often uses the definite article when talking about a whole class or type of thing in a general sense.

So:

Ο σολομός είναι ακριβός
literally looks like The salmon is expensive, but it really means Salmon is expensive or Salmon is an expensive fish.

This is very common in Greek. For example:

Ο σκύλος είναι πιστός = Dogs are loyal / The dog is loyal as a species

So the article here does not mean one specific salmon.

Because ακριβός has to agree with σολομός.

σολομός is:

• masculine
• singular
• nominative

So the adjective also becomes:

• masculine
• singular
• nominative

That gives ακριβός.

Compare:

• ο σολομός είναι ακριβός = masculine
• το ψάρι είναι ακριβό = neuter
• η βρώμη είναι ακριβή = feminine

Greek adjectives change form to match the noun.

γι’ αυτό means that’s why, for that reason, or so.

In this sentence, it connects the first idea to the result:

Ο σολομός είναι ακριβός, γι’ αυτό...
= Salmon is expensive, so...

It introduces a consequence, not a cause.

No. They are related in meaning, but they do different jobs.

• γι’ αυτό = that’s why / so / for that reason
• γιατί = because or why

Examples:

• Ο σολομός είναι ακριβός, γι’ αυτό δεν τον αγοράζω συχνά.
  = Salmon is expensive, so I don’t buy it often.
• Δεν τον αγοράζω συχνά γιατί είναι ακριβός.
  = I don’t buy it often because it’s expensive.

So in your sentence, γι’ αυτό is correct because the second part is the result.

It is a shortened form of για αυτό.

Before a vowel, Greek often contracts this in writing:

• για αυτό → γι’ αυτό

This is very common and natural. You should recognize both, but γι’ αυτό is the standard written form in this kind of sentence.

Because Greek usually does not need subject pronouns when the verb ending already shows the person.

τρώω already means I eat.

So:

• τρώω = I eat
• τρώμε = we eat
• τρώει = he/she/it eats

Greek is a pro-drop language, so εγώ is often omitted unless you want emphasis or contrast.

For example:

• Συνήθως τρώω βρώμη = I usually eat oatmeal.
• Εγώ τρώω βρώμη, αλλά αυτός τρώει αυγά = I eat oatmeal, but he eats eggs.

συνήθως means usually.

Its position is fairly flexible, but placing it before the verb is very natural and common:

συνήθως τρώω βρώμη = I usually eat oatmeal

You could also hear:

• τρώω συνήθως βρώμη
• βρώμη τρώω συνήθως
  but these may sound a little more marked or shift the focus.

So γι’ αυτό συνήθως τρώω... is a very normal word order.

Because βρώμη here is being used as a general food item, not as a specific previously mentioned portion of oats.

Greek often leaves out the article with food or mass nouns when the meaning is indefinite or general:

• τρώω βρώμη = I eat oats / oatmeal
• πίνω νερό = I drink water
• τρώω ψωμί = I eat bread

If you said τη βρώμη, it would more likely sound like a specific oatmeal that has already been identified in the conversation.

It can depend on context.

Literally, βρώμη is oats. But in everyday context, especially with breakfast and τρώω, it can naturally refer to oatmeal / oats as food.

So in this sentence, an English speaker would most likely understand it as:

• I usually eat oatmeal in the morning or
• I usually eat oats in the morning

The exact English translation depends on the situation more than on the Greek word itself.

Because ψάρι in the singular often works like the English food word fish.

So:

• τρώω ψάρι = I eat fish

This is the normal way to talk about fish as food in general.

If you said ψάρια, that would more strongly suggest:

• different fish
• several fish
• fish in a more countable sense

In a sentence about diet or eating habits, ψάρι is the most natural choice.

These are standard Greek time expressions.

• το πρωί = in the morning
• το Σαββατοκύριακο = on the weekend

Greek often uses the article in expressions of time where English uses a preposition.

So although το is the article the, the whole phrase functions adverbially:

• το πρωί does not mean only the morning in a literal noun sense here; it means in the morning
• το Σαββατοκύριακο means on the weekend

These are very common set patterns that you should learn as complete expressions.

Because Greek often expresses time directly without a separate preposition where English needs one.

So Greek says:

• το πρωί = in the morning
• το βράδυ = in the evening / at night
• το Σάββατο = on Saturday
• το Σαββατοκύριακο = on the weekend

This is just a normal feature of Greek time expressions. English needs in or on, but Greek often does not.

It is singular.

το Σαββατοκύριακο means the weekend.

Even though it comes from the idea of Saturday + Sunday, it is treated as one neuter singular noun.

So:

• το Σαββατοκύριακο = the weekend
• τα Σαββατοκύριακα = weekends

Because it is limiting the time, not the food.

ψάρι μόνο το Σαββατοκύριακο means: fish only on the weekend

The idea is: I eat fish, but only at that time.

Compare:

• τρώω μόνο ψάρι = I eat only fish
  here μόνο limits ψάρι
• τρώω ψάρι μόνο το Σαββατοκύριακο = I eat fish only on the weekend
  here μόνο limits το Σαββατοκύριακο

So the position of μόνο helps show what is being restricted.

Because σολομός is a specific kind of fish, while ψάρι is more general.

The speaker first gives the reason:

• Ο σολομός είναι ακριβός = Salmon is expensive

Then they talk about their eating habit more broadly:

• και ψάρι μόνο το Σαββατοκύριακο = and fish only on the weekend

So the sentence does not necessarily mean they eat salmon only on weekends. It means salmon is expensive, and as a result their fish-eating is limited in general.

If the speaker wanted to be very specific, they could say: και σολομό μόνο το Σαββατοκύριακο
= and salmon only on the weekend

But ψάρι sounds broader and more natural in a general diet statement.