Η ιατρική αλλάζει γρήγορα.

Here it functions as a noun: medicine or medical science.

Historically, ιατρική is also the feminine form of the adjective ιατρικός (medical), but Greek often uses adjectives as nouns when the meaning is clear. So η ιατρική means medicine as a field.

Because Greek uses the definite article much more often than English.

In this sentence, η is the feminine singular nominative article, and η ιατρική is the normal way to say medicine as a subject or field of study. Greek commonly does this with abstract nouns and academic disciplines.

So even though English says just medicine, Greek naturally says the medicine in form: η ιατρική.

It is feminine singular nominative.

• η = feminine singular nominative definite article
• ιατρική = feminine singular nominative form

It is in the nominative because it is the subject of the sentence: the thing that is changing.

Αλλάζει is the third-person singular present form of αλλάζω (to change).

So grammatically it means:

• he/she/it changes
• or is changing

In this sentence, the subject is η ιατρική, which is singular, so αλλάζει is singular too.

It can mean either, depending on context.

The Greek present tense often covers both:

• a general truth or ongoing situation: changes
• something happening now or in the current period: is changing

So this sentence could be understood as:

• Medicine changes quickly
• Medicine is changing quickly

Both are natural translations.

Because in Modern Greek, verbs agree with the subject in person and number, not in gender.

So αλλάζει is used for any third-person singular subject:

• η ιατρική αλλάζει = medicine changes
• ο κόσμος αλλάζει = the world changes
• το σύστημα αλλάζει = the system changes

The subject’s gender does not affect the verb form.

Because γρήγορα is functioning as an adverb, meaning quickly, and it modifies the verb αλλάζει.

• γρήγορη would be an adjective meaning quick, used with a feminine noun
• γρήγορα means quickly, describing how the changing happens

Compare:

• η γρήγορη αλλαγή = the quick change
• αλλάζει γρήγορα = changes quickly

So here γρήγορα is the correct form.

Yes. Αλλάζω can be both:

• transitive: changing something
• intransitive: undergoing change

In this sentence it is intransitive:

• Η ιατρική αλλάζει γρήγορα. = Medicine changes quickly.

A transitive example would be:

• Αλλάζω γνώμη. = I change my mind.

Not completely. Greek word order is more flexible than English.

Η ιατρική αλλάζει γρήγορα is the most neutral, straightforward order:

• subject + verb + adverb

But Greek can move elements around for emphasis. For example:

• Γρήγορα αλλάζει η ιατρική puts more emphasis on γρήγορα

Even so, the original sentence is the most natural choice for a neutral statement.

Because Η without an accent is the definite article the, while ή with an accent means or.

So:

• Η ιατρική = the medicine / medicine
• ή = or

At the beginning of the sentence, the article is simply capitalized: Η. The accent makes a real difference in Greek, even in very short words.