Ο μύκητας εμφανίζεται όταν υπάρχει υγρασία.

ο is the masculine singular definite article, meaning the.

Greek often uses the definite article more regularly than English, especially when talking about a thing as a general category or as the known subject of the sentence. So ο μύκητας is the natural way to say the fungus / the mold here.

Without the article, μύκητας would sound more like a fungus or just a dictionary-style noun, depending on context.

It is in the nominative singular.

That is because ο μύκητας is the subject of the verb εμφανίζεται. In other words, it is the thing that appears.

• ο = nominative masculine singular article
• μύκητας = nominative masculine singular noun

Because it is a masculine noun of a very common Greek type.

Many masculine nouns in Modern Greek end in -ας in the nominative singular. For example:

• ο άντρας = the man
• ο μαθητής = the student
• ο μύκητας = the fungus

So the ending -ας helps signal that the noun is masculine singular nominative.

εμφανίζεται is the 3rd person singular present tense form of εμφανίζομαι.

Here it means something like:

• appears
• shows up
• becomes visible / develops

So ο μύκητας εμφανίζεται means that the fungus/mold appears or shows up.

Grammatically, this verb is in the middle/passive form in Modern Greek, but in meaning it is often active-like in English. This is very common in Greek.

That is just how this verb works in Modern Greek.

The dictionary form is εμφανίζομαι, not usually εμφανίζω for this meaning. Greek has many verbs that use middle/passive endings but are translated with active verbs in English.

So:

• εμφανίζομαι = I appear
• εμφανίζεται = he/she/it appears

You do not need to translate it as a passive like is appeared. In this sentence, it simply means appears.

Because Greek normally does not use subject pronouns unless they are needed for emphasis or contrast.

The verb ending already tells you the subject is he/she/it:

• εμφανίζομαι = I appear
• εμφανίζεσαι = you appear
• εμφανίζεται = he/she/it appears

Since ο μύκητας is already there as the subject, no extra pronoun is needed.

όταν means when.

It introduces a time clause:

• όταν υπάρχει υγρασία = when there is moisture / humidity

In Greek, όταν is followed by a normal finite verb. That is exactly what happens here with υπάρχει.

This structure is very common:

• Όταν βρέχει, μένω σπίτι. = When it rains, I stay home.
• Όταν υπάρχει υγρασία, εμφανίζεται ο μύκητας. = When there is moisture, fungus appears.

Because υπάρχει means there is / there exists, which is the natural Greek way to express existence or presence.

So:

• υπάρχει υγρασία = there is moisture / humidity present

By contrast, είναι means is, and it links a subject with a description or identity. It would not work naturally here.

Compare:

• Υπάρχει πρόβλημα. = There is a problem.
• Είναι πρόβλημα. = It is a problem.

So υπάρχει is the correct verb in this sentence.

Because υγρασία is singular.

The verb agrees with the noun that follows:

• υπάρχει υγρασία = there is moisture
• υπάρχουν λεκέδες = there are stains

Since υγρασία is a singular noun, υπάρχει must also be singular.

It is in the nominative singular.

In the clause υπάρχει υγρασία, the noun υγρασία is the thing that exists, so it functions as the grammatical subject.

Even though English uses there is, Greek does not use a dummy subject like there. Greek simply says:

• exists moisture
  or more naturally
• there is moisture

So υγρασία stays in the nominative.

Because Greek often leaves out the article with abstract nouns or when speaking generally.

Here, υγρασία means moisture/humidity in a general sense, not a specific, already identified moisture. So:

• υπάρχει υγρασία = there is moisture / humidity

If you added the article, η υγρασία, it would sound more specific, like the moisture or the humidity in a particular context.

Because it expresses a general truth or regular occurrence.

Greek, like English, often uses the present tense for things that happen habitually or under certain conditions:

• Ο μύκητας εμφανίζεται όταν υπάρχει υγρασία.
  = Fungus appears when there is moisture.

This does not mean it is happening only right now. It means this is what generally happens.

Yes, Greek word order is fairly flexible.

You could also say:

• Όταν υπάρχει υγρασία, εμφανίζεται ο μύκητας.

This version puts the when-clause first, which may sound slightly more natural in many contexts.

Both versions are correct. The difference is mostly one of emphasis and flow, not basic meaning.

A simple pronunciation guide is:

O MEE-kee-tas eh-fah-NEE-zeh-teh OH-tan ee-PAR-khee ee-grah-SEE-ah

A few helpful notes:

• μύκητας has the stress on the first syllable: ΜΥκητας
• εμφανίζεται has the stress on -νί-: εμφανΙζεται
• υπάρχει has the stress on -πάρ-: υΠΑΡχει
• υγρασία has the stress on -σί-: υγραΣΙα

Also:

• γρ in υγρασία is pronounced with a soft Greek g sound before r
• χ in υπάρχει is the Greek sound heard in Scottish loch or German Bach

It can depend on context.

μύκητας literally means fungus, but in everyday contexts it may refer to things that English speakers would call:

• fungus
• mold
• sometimes mildew, depending on the situation

So the exact English word depends on what kind of growth is being talked about. In this sentence, if the context is walls, bathrooms, or damp areas, English might naturally use mold.