Άκουσα μια δυνατή βροντή και είδα μια αστραπή πάνω από το βουνό.

Both are in the aorist past, which is the normal Greek tense for a single completed event in the past.

• άκουσα = I heard
• είδα = I saw

In this sentence, the speaker is reporting two finished events: first hearing the thunder, then seeing the lightning.

A very important detail:

• ακούω → άκουσα
• βλέπω → είδα

So είδα is an irregular aorist form of βλέπω.

Because the verb ending already shows the subject.

• άκουσα = I heard
• είδα = I saw

So Greek usually does not need the pronoun εγώ unless you want emphasis or contrast.

For example:

• Άκουσα μια δυνατή βροντή. = I heard a loud thunderclap.
• Εγώ άκουσα μια δυνατή βροντή. = I heard a loud thunderclap.
  (with extra emphasis on I)

This is very normal in Greek.

Μια is the feminine singular form of the indefinite article, meaning a / an.

Both nouns are feminine:

• η βροντή = the thunder / thunderclap
• η αστραπή = the lightning flash

So in the accusative singular, after the verbs, you get:

• μια βροντή
• μια αστραπή

This is why you do not use ένας or ένα here.

Quick comparison:

• ένας άντρας = a man
• μια γυναίκα = a woman
• ένα παιδί = a child

Because adjectives in Greek must agree with the noun in gender, number, and case.

Here:

• βροντή is feminine
• it is singular
• it is the object of the verb, so it is in the accusative

So the adjective must match:

• μια δυνατή βροντή

The adjective δυνατός changes form:

• δυνατός = masculine
• δυνατή = feminine
• δυνατό = neuter

So δυνατή is the correct feminine singular form here.

It can mean thunder, but in a sentence like this it often feels like a clap of thunder or a thunderclap, because it is something heard as a single event.

So:

• άκουσα μια βροντή often sounds like I heard a thunderclap
• άκουσα βροντές = I heard thunder / thunderclaps

English and Greek do not always divide this idea in exactly the same way, so both thunder and thunderclap may be good translations depending on context.

That is a very good question. In Greek, αστραπή is often treated as a countable event: a flash of lightning.

So:

• μια αστραπή = a lightning flash / a flash of lightning

This is very natural Greek. Even if English often just says lightning, Greek commonly uses αστραπή as a single visible flash.

It means above the mountain or over the mountain.

The phrase is:

• πάνω από = above / over
• το βουνό = the mountain

So the lightning was seen in the sky over or above the mountain.

A useful distinction:

• πάνω από = above, over
• πάνω σε = on, on top of

So:

• πάνω από το βουνό = above the mountain
• πάνω στο βουνό = on the mountain

That difference is important.

Because από takes the accusative in Modern Greek, and βουνό is a neuter noun.

The basic forms are:

• το βουνό = the mountain
• nominative and accusative are the same here

So after πάνω από, you get:

• πάνω από το βουνό

This is completely regular.

Greek word order is more flexible than English, because the verb endings and noun forms already give a lot of grammatical information.

The sentence as written is very natural:

• Άκουσα μια δυνατή βροντή και είδα μια αστραπή πάνω από το βουνό.

But other orders are possible for emphasis, for example:

• Μια δυνατή βροντή άκουσα και είδα μια αστραπή πάνω από το βουνό.
• Πάνω από το βουνό είδα μια αστραπή.

These alternatives may sound more marked or emphasize different parts of the sentence. The original version is the most neutral and straightforward.

The written accents show the stressed syllable.

• Άκουσα → stress on Ά
• μια → usually light, often unstressed in fast speech
• δυνατή → stress on the last syllable
• βροντή → stress on the last syllable
• και → usually like keh
• είδα → stress on εί
• αστραπή → stress on the last syllable
• πάνω → stress on πά
• από → stress on πό
• βουνό → stress on the last syllable

A rough pronunciation guide:

AH-koo-sa mia thee-na-TEE vrohn-TEE keh EE-tha mia a-stra-PEE PA-no a-PO to voo-NO

A few notes:

• δ sounds like th in this
• β sounds like v
• θ sounds like th in think
• ου sounds like oo
• αι in και sounds like e

Yes, but that would change the meaning.

Your sentence uses the aorist:

• άκουσα
• είδα

This presents the actions as single completed events.

If you used the imperfect:

• άκουγα
• έβλεπα

it would suggest something more ongoing, repeated, or backgrounded.

Compare:

• Άκουσα μια δυνατή βροντή. = I heard a loud thunderclap.
  (one completed event)

• Άκουγα βροντές. = I was hearing thunder / I kept hearing thunder.
  (ongoing or repeated)

• Είδα μια αστραπή. = I saw a flash of lightning.
• Έβλεπα αστραπές. = I was seeing flashes of lightning / I kept seeing lightning.

So the forms in your sentence are the natural choice for a specific moment in the past.

Here και simply means and. It links the two past actions:

• Άκουσα μια δυνατή βροντή
• και
• είδα μια αστραπή πάνω από το βουνό

So the sentence tells a sequence of events: the speaker heard thunder and saw lightning.

In speech, και can sometimes also mean things like also, even, or be used in various expressions, but here it is just the normal coordinating and.