Learn Hardcore Greek: Το καλοκαίρι πάω διακοπές σε ένα ελληνικό νησί. - In the summer I go on vacation on a Greek island.

Το καλοκαίρι literally is the summer, but in context it means in (the) summer / in the summer.
Greek often drops a separate preposition like in here; the combination of article + noun is enough.

Some common parallels:

Το πρωί = in the morning
Το βράδυ = in the evening
Τον χειμώνα = in (the) winter

So Το καλοκαίρι at the beginning of the sentence functions like an adverbial time phrase: In the summer, …

What case is το καλοκαίρι in, and what is its grammatical role?

Καλοκαίρι is a neuter noun; το is the neuter singular definite article.

Formally, το καλοκαίρι is neuter singular, and the form is the same for nominative and accusative. In this sentence it functions as an adverbial time expression, not as the subject.

So:

Case form: neuter singular (nominative/accusative form)
Role: time adverbial meaning in the summer / during the summer

Why is there no I in the Greek sentence? Where is the subject?

The subject I is included in the verb ending.

πάω is 1st person singular: I go.
Greek is a pro-drop language: subject pronouns are often omitted because the verb ending shows the person and number.

You could say Εγώ το καλοκαίρι πάω διακοπές…, but that puts extra emphasis on I (as in I, in the summer, go on vacation…). In neutral speech, you simply say Το καλοκαίρι πάω….

What is the difference between πάω and πηγαίνω?

Both mean to go, and in many situations they can replace each other.

πάω is more colloquial and very common in everyday speech.
πηγαίνω is a bit more formal or neutral; it is the more complete verb paradigm and is often what you see in dictionaries.

In this sentence, you could also say:

Το καλοκαίρι πηγαίνω διακοπές σε ένα ελληνικό νησί.

Meaning remains the same: In the summer I go on vacation to a Greek island.

The verb is present tense (πάω). Does it mean something I do every summer, or this coming summer?

The Greek simple present can express:

Habitual action
- Το καλοκαίρι πάω διακοπές σε ένα ελληνικό νησί.
  = Every summer I go on vacation to a Greek island.
Near future / planned action, especially with a time expression
- Same sentence in a certain context can mean
  This summer, I’m going on vacation to a Greek island.

Context usually tells you whether it is a general habit or a specific plan. Without more context, the habitual reading is the most natural.

Why do we say πάω διακοπές without a preposition or article before διακοπές?

Πάω διακοπές is a fixed expression in Greek meaning I go on vacation / I go on holiday.

διακοπές is plural and here it is used without an article:
- πάω διακοπές = I go on vacation
- πήγα διακοπές = I went on vacation

This is similar to English patterns like:

go home, go camping, go shopping, where there is no preposition before home, camping, shopping.

If you add an article, you change the nuance:

Οι διακοπές = the vacation(s) as a noun phrase, for example:
- Οι διακοπές μου ήταν υπέροχες. = My vacation was wonderful.

Why is διακοπές plural? In English we say vacation in the singular.

In Greek, διακοπές (from διακοπή = interruption) is normally used in the plural when referring to holidays or vacation time.

Οι διακοπές literally = the breaks or the interruptions, but idiomatically it corresponds to English holiday / vacation.
This plural is the standard form for the concept of vacation, just like English holidays in British English.

Some examples:

Πότε έχεις διακοπές; = When do you have vacation / holidays?
Πάμε διακοπές τον Αύγουστο. = We go on vacation in August.

What does σε mean in σε ένα ελληνικό νησί, and can it be contracted?

Σε is a very common preposition in Greek. Its basic meaning is in / at / to, depending on context.

In σε ένα ελληνικό νησί it corresponds to to:

σε ένα ελληνικό νησί = to a Greek island

In speech and informal writing, σε often contracts with the following article:

σε + ένα → σ’ ένα
So you will often see/hear:
- Το καλοκαίρι πάω διακοπές σ’ ένα ελληνικό νησί.

Both σε ένα and σ’ ένα are correct; the contracted form is just more colloquial and natural in fast speech.

Why is it ένα ελληνικό νησί and not ένας or μία?

Greek has three genders: masculine, feminine, neuter. The articles and adjectives must agree in gender, number, and case with the noun.

νησί (island) is neuter.
The matching indefinite article in the accusative singular is ένα (same form for nominative and accusative).
The adjective ελληνικό is also neuter singular accusative.

So:

σε ένα ελληνικό νησί
- ένα: neuter sing.
- ελληνικό: neuter sing.
- νησί: neuter sing.

ένας (masc.) and μία/μια (fem.) would be wrong because they do not match the neuter noun νησί.

Why is the adjective in ένα ελληνικό νησί placed before the noun? Can it go after?

The normal, neutral order in Greek is:

article + adjective + noun

So:

ένα ελληνικό νησί = a Greek island
This is the most natural way to say it.

You can put an adjective after the noun in some constructions, but usually that involves repeating the article or changing the emphasis, for example:

ένα νησί ελληνικό might sound stylistic or poetic, or emphasize Greek as a characteristic.
Very common alternative pattern: το νησί το ελληνικό, but that is more marked in style and emphasis.

For everyday speech, ένα ελληνικό νησί is the standard order.

Why do we use σε here and not something like για or προς?

In modern Greek, σε is the default preposition for:

location: in, at, on
destination: to

In the sense of travelling to a place, σε is the normal preposition:

πάω στο σχολείο = I go to school
πάω στο σπίτι = I go home
πάω σε ένα νησί = I go to an island

για sometimes corresponds to for or to (in order to), but with destinations you usually still use σε:

Πάω για διακοπές σε ένα ελληνικό νησί.
Here για διακοπές = for vacation, and σε ένα ελληνικό νησί = to a Greek island.

προς can mean towards, but it is less common in everyday speech and has a slightly more directional or formal feel; σε is the natural choice here.

How is the sentence pronounced, especially the vowels and stresses?

Approximate pronunciation with stressed syllables in capitals:

Το καλοκαίρι → to ka-lo-KÉ-ri
- αι in καλοκαίρι sounds like e in bed.
- Stress on κέ.
πάω → PÁ-o
- Two syllables: πά-ω (often flows together in fast speech, like páo).
διακοπές → dhia-ko-PÉS
- δι before a vowel is often pronounced like ðia (soft th plus ya), but many speakers say something close to δya.
- ε is like e in get.
- Stress on πές.
σε ένα → se É-na
- Often contracted in everyday speech to σ’ ένα: sÉ-na.
ελληνικό → e-li-ni-KÓ
- Stress on κό.
- Double λ just lengthens the l a bit.
νησί → ni-SÍ
- η and ι both sound like i in machine.
- Stress on σί.

All together:

Το καλοκαίρι πάω διακοπές σε ένα ελληνικό νησί.
→ to ka-lo-KÉ-ri PÁ-o dhia-ko-PÉS se É-na e-li-ni-KÓ ni-SÍ.