Άφησα τα ρούχα στο μπαλκόνι όλη νύχτα, αλλά το πρωί δεν ήταν τελείως στεγνά.

Άφησα means I left.

It is the 1st person singular aorist of αφήνω (to leave).
The aorist is commonly used for a completed action in the past, so Άφησα τα ρούχα... means I left the clothes...

Τα ρούχα means the clothes.

• τα = the for neuter plural
• ρούχα = clothes

The noun ρούχα is neuter plural, so it takes the article τα.
A useful comparison:

• το ρούχο = the garment / the item of clothing (singular)
• τα ρούχα = the clothes (plural)

Because adjectives in Greek usually agree with the noun they describe in gender, number, and case.

Since ρούχα is neuter plural, the adjective must also be neuter plural:

• τα ρούχα
• στεγνά

So στεγνά matches ρούχα.

Στο is a contraction of σε + το.

• σε = in / on / at / to
• το = the (neuter singular)

So:

• σε το μπαλκόνι → στο μπαλκόνι

Here it means on the balcony.

Both can exist, but όλη νύχτα is a very natural adverbial time expression meaning all night.

Greek often omits the article in certain time expressions, especially when the phrase functions adverbially.

So:

• όλη νύχτα = all night
• όλη τη νύχτα = also possible, a bit more explicitly the whole night

In this sentence, όλη νύχτα sounds very natural.

Because το πρωί is a fixed and very common time expression meaning in the morning / in the morning time.

Greek often uses the article in expressions like:

• το πρωί = in the morning
• το βράδυ = in the evening / at night
• το μεσημέρι = at noon / midday

So even though English usually says just in the morning, Greek normally uses the here.

Δεν ήταν means they were not.

This describes the state of the clothes in the morning: they were not completely dry.

• ήταν = imperfect of είμαι (to be)
• It is used here to describe a past situation or condition.

If you used έγιναν (became), the meaning would shift more toward they did not become completely dry, which is slightly different.
The sentence as written focuses on their condition that morning.

This is a very common question.

In Modern Greek, the verb είμαι often uses the 3rd person singular form with a neuter plural subject, especially in everyday language.

So:

• τα ρούχα ήταν στεγνά

is normal and correct.

Even though ρούχα is plural, ήταν is the usual form here.

Τελείως means completely / totally.

It is an adverb, and it modifies the adjective στεγνά:

• στεγνά = dry
• τελείως στεγνά = completely dry

So δεν ήταν τελείως στεγνά means they were not completely dry.

Because Greek often omits subject pronouns when they are understood from the context.

Here, we already know the sentence is talking about τα ρούχα (the clothes), so Greek does not need to repeat they.

English says:

• they were not completely dry

Greek naturally says:

• δεν ήταν τελείως στεγνά

with the subject understood from the earlier noun.

Greek word order is more flexible than English word order, but the original order is very natural.

• Άφησα τα ρούχα στο μπαλκόνι όλη νύχτα, αλλά το πρωί δεν ήταν τελείως στεγνά.

This flows as:

1. action: I left the clothes
2. place: on the balcony
3. time: all night
4. contrast: but
5. later time: in the morning
6. result/state: they were not completely dry

You could sometimes move phrases around for emphasis, but this version sounds normal and clear.

Αλλά means but.

It introduces a contrast:

• I left the clothes on the balcony all night,
• but in the morning they were not completely dry.

So it connects the expectation (they might be dry) with the actual result (they were not completely dry).

Στεγνός / στεγνή / στεγνό basically means dry, and like English, it can be used in different contexts.

In this sentence, because it refers to ρούχα, it clearly means dry as in not wet.

So:

• στεγνά ρούχα = dry clothes

That is the straightforward meaning here.