Η αδερφή μου έχει αλλεργία στη σκόνη, γι’ αυτό φτερνίζεται όταν το δωμάτιο δεν είναι καθαρό.

Greek usually keeps the definite article with family members and with possessives. So η αδερφή μου is the normal way to say my sister.

Literally, it is closer to the sister of mine:

• η = the
• αδερφή = sister
• μου = my

Dropping the article is possible in some special contexts, but η αδερφή μου is the standard neutral phrasing.

The short possessive forms in Greek, such as μου, σου, του, της, normally come after the noun:

• η αδερφή μου = my sister
• το βιβλίο σου = your book

This is one of the first word-order differences English speakers notice. In everyday Greek, post-position is the default for these unstressed possessives.

Yes. Both mean sister.

• αδερφή is very common in everyday Modern Greek.
• αδελφή is also correct, and may sound a bit more formal or traditional.

A learner will often hear and see both, so it is good to recognize them as variants of the same word.

Yes. Greek commonly uses έχω + noun for physical conditions and symptoms, where English often uses have too, but not always in exactly the same way.

So:

• έχει αλλεργία = she has an allergy / she is allergic

In this sentence, Greek expresses the idea through the noun αλλεργία rather than an adjective like allergic.

Because Greek often leaves out the article with certain condition nouns after έχω.

So έχει αλλεργία sounds natural as a general statement. If you added an article or another determiner, it could make the meaning more specific, but here the plain noun is the normal choice.

στη is the contracted form of σε + τη(ν).

So:

• σε = to, in, on, at
• τη(ν) = the, feminine singular accusative
• στη = to/in/on the

Here:

• στη σκόνη = to dust / to the dust

With αλλεργία, Greek uses σε for what someone is allergic to, so αλλεργία στη σκόνη means allergy to dust.

Greek uses the definite article more often than English does. After prepositions, and especially with general substances or categories, the article is very common.

So στη σκόνη is the natural Greek way to express to dust. You should not expect article usage to match English word for word.

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

It comes from για αυτό, and the apostrophe shows contraction:

• για αυτό → γι’ αυτό

In normal writing, the contracted form is very common.

Because this verb is one of many Greek verbs that use middle/passive-style forms but have an active meaning in English.

The dictionary form is φτερνίζομαι = I sneeze
So:

• φτερνίζεται = he/she sneezes

This does not mean is sneezed. It is simply how this verb is conjugated.

όταν means when / whenever, while αν means if.

Here the sentence describes a repeated, usual situation:

• whenever the room is not clean, she sneezes

So όταν is the right choice. If you used αν, the sentence would sound more like a condition: if the room is not clean.

Because the sentence describes a habitual or repeated fact, not a one-time event.

Greek often uses the present tense for general truths and repeated actions:

• φτερνίζεται = she sneezes
• όταν το δωμάτιο δεν είναι καθαρό = when the room is not clean

So the meaning is basically she tends to sneeze whenever the room isn’t clean.

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

Greek nouns have grammatical gender, and the article must match:

• ο for many masculine nouns
• η for many feminine nouns
• το for neuter nouns

So:

• το δωμάτιο = the room

Because καθαρό has to agree with το δωμάτιο, which is neuter singular.

After είναι, Greek adjectives still agree with the noun they describe:

• το δωμάτιο είναι καθαρό = the room is clean

Compare:

• η κουζίνα είναι καθαρή = the kitchen is clean
• ο κήπος είναι καθαρός = the garden is clean

In Greek, the negative particle δεν goes directly before the verb in ordinary statements.

So:

• είναι = is
• δεν είναι = is not

This is the normal way to make a verb negative in Modern Greek.

Yes. Greek word order is more flexible than English word order, because endings and articles give a lot of grammatical information.

This sentence uses a very natural neutral order:

• Η αδερφή μου έχει αλλεργία στη σκόνη, γι’ αυτό φτερνίζεται όταν το δωμάτιο δεν είναι καθαρό.

But Greek can move parts around for emphasis. For example, όταν το δωμάτιο δεν είναι καθαρό could also appear in a different position if the speaker wanted a different emphasis. The version here is simply the most straightforward one for a learner.