Η υγρασία στο μπάνιο είναι μεγάλη, γι’ αυτό ανοίγω το παράθυρο.

Στο is a contraction of σε + το.

• σε = in / at / to
• το = the for a neuter noun

So:

• σε το μπάνιο → στο μπάνιο

This contraction is completely standard in modern Greek.

Because υγρασία is a feminine noun in Greek, so it takes the feminine singular article η.

Greek nouns have grammatical gender, and the article has to match the noun:

• ο = masculine
• η = feminine
• το = neuter

So:

• η υγρασία = the humidity

Because it has to agree with υγρασία, which is feminine singular.

The adjective μεγάλος / μεγάλη / μεγάλο means big / great / large, and its form changes to match the noun’s gender and number:

• masculine: μεγάλος
• feminine: μεγάλη
• neuter: μεγάλο

Since υγρασία is feminine singular, Greek uses μεγάλη.

Literally, μεγάλη means big / large / great, but in this sentence it means something like high or a lot of humidity.

Greek often uses words more flexibly than English does. So η υγρασία είναι μεγάλη is understood as the humidity is high / there is a lot of humidity.

You may also hear:

• υψηλή υγρασία = high humidity
• πολλή υγρασία = a lot of humidity

But μεγάλη works and is understandable.

Είναι μεγάλη is the neutral, standard word order: is high / is great.

Greek word order is more flexible than English word order, but the usual pattern here is:

• subject: Η υγρασία
• place phrase: στο μπάνιο
• verb: είναι
• adjective: μεγάλη

You could change the order for emphasis, but είναι μεγάλη is the most natural basic structure.

Γι’ αυτό means for this reason, that’s why, or therefore.

It links the first idea to the result:

• humidity is high
• therefore / that’s why I open the window

It is very common in everyday Greek.

Because it is a shortened form of για αυτό.

In speech, Greek often shortens για before a following vowel, and this is shown in writing with an apostrophe:

• για αυτό → γι’ αυτό

Both relate to the same expression, but γι’ αυτό is the usual written form in this meaning.

It is written as two parts:

• γι’
• αυτό

But together they function as a fixed expression meaning that’s why / therefore.

So when reading, you should understand it as one unit of meaning.

Ανοίγω is the present tense, first person singular: I open / I am opening.

In Greek, the present tense can describe:

• something happening now
• something someone does generally or habitually

So this sentence can mean either:

• I’m opening the window now, because the bathroom is humid
• I open the window when the bathroom is humid

The exact interpretation depends on context.

Because παράθυρο is a neuter noun, so it takes the neuter singular article το.

So:

• το παράθυρο = the window

It is also in the accusative case here, because it is the direct object of ανοίγω. But for neuter singular nouns like this, the nominative and accusative forms are the same, so you do not see a change.

Greek does not need to repeat the object if the noun is already stated.

Here Greek simply says:

• ανοίγω το παράθυρο = I open the window

If the speaker wanted to say I open it, Greek could use a pronoun, but in this sentence the full noun is used instead.

Yes, it is normal and helpful here.

The comma separates:

• the cause: Η υγρασία στο μπάνιο είναι μεγάλη
• the result: γι’ αυτό ανοίγω το παράθυρο

This is similar to English punctuation in a sentence like The humidity in the bathroom is high, so I open the window.

Yes. Greek word order is fairly flexible, especially for emphasis.

For example, you could also say:

• Γι’ αυτό ανοίγω το παράθυρο.
• Το παράθυρο ανοίγω γι’ αυτό. — possible, but marked and less neutral
• Στο μπάνιο η υγρασία είναι μεγάλη, γι’ αυτό ανοίγω το παράθυρο.

The original version is natural and neutral.

Greek often uses είναι (is) where English might prefer there is or a different phrasing.

So:

• Η υγρασία στο μπάνιο είναι μεγάλη literally = The humidity in the bathroom is great/high
• natural English = The bathroom is very humid / There is a lot of humidity in the bathroom

Greek and English do not always package the same idea in exactly the same way.

You could move στο μπάνιο earlier for emphasis:

• Στο μπάνιο η υγρασία είναι μεγάλη, γι’ αυτό ανοίγω το παράθυρο.

This gives extra focus to in the bathroom. Greek allows this kind of movement more easily than English does.

It sounds like normal, standard modern Greek. It is perfectly fine in everyday use.

A speaker might say something slightly more conversational, such as:

• Έχει πολλή υγρασία στο μπάνιο, γι’ αυτό ανοίγω το παράθυρο.

But the original sentence is grammatical, natural, and clear.