Βρέχει ασταμάτητα από το πρωί.

It can mean both. The form βρέχει is the present tense of βρέχω, and Greek present tense can describe either:

• a general/habitual action: It rains
• an action happening now: It is raining

In this sentence, από το πρωί makes it clear that the meaning is ongoing: it has been raining since morning / since this morning.

The dictionary form is βρέχω.

This verb has two common uses:

• impersonal/weather use: βρέχει = it is raining
• ordinary transitive use: βρέχω κάτι = I wet something

So in this sentence, it is being used in the weather sense.

This is a very common Greek pattern. Greek often uses the present tense for something that started in the past and is still continuing now, especially with a time phrase like από το πρωί.

So:

• Βρέχει από το πρωί
  literally looks like It is raining since morning
• but in natural English it is usually translated as It has been raining since this morning

English prefers the present perfect progressive, while Greek often just uses the present.

Ασταμάτητα is an adverb. It describes how it is raining: nonstop, continuously, without stopping.

Because it is an adverb, it does not change form for gender, number, or case.

It is related to the adjective ασταμάτητος / ασταμάτητη / ασταμάτητο, meaning unstoppable or unceasing.

In time expressions, από can mean since when it shows the starting point of an action.

So:

• από το πρωί = since morning / since this morning

Here it tells us when the rain began and implies that it is still continuing.

After από, Greek uses the accusative case.

So το πρωί here is accusative. Since πρωί is a neuter singular noun, its nominative and accusative forms look the same, so you do not see a visible change.

The article το is natural because Greek often uses the article with parts of the day:

• το πρωί = the morning / in the morning / this morning
• το βράδυ = the evening / tonight
• το μεσημέρι = at noon / midday

So από το πρωί is the normal way to say since this morning / since morning.

No. Greek word order is fairly flexible.

This version is natural and neutral:

• Βρέχει ασταμάτητα από το πρωί.

But you could also say:

• Από το πρωί βρέχει ασταμάτητα.

That puts more emphasis on since morning.

You may also hear other orders in speech, depending on what the speaker wants to emphasize. The meaning stays basically the same, but the focus changes.

Not exactly. They are similar, but not the same.

• από το πρωί = since this morning, starting then and continuing now
• όλο το πρωί = all morning, during the whole morning period

So:

• Βρέχει ασταμάτητα από το πρωί
  suggests it started in the morning and is still raining now.

• Έβρεχε ασταμάτητα όλο το πρωί
  means It was raining nonstop all morning, but it may have stopped later.

That difference is important.

A rough pronunciation guide is:

VREH-hi a-sta-MA-ti-ta a-PO to pro-EE

A few useful notes:

• β sounds like English v, not b
• χ in βρέχει is not a hard English k sound; before ει it is a softer sound, somewhat like the ch in German ich
• the stress falls on:
  • ΒΡΕ-χει
  • ασταΜΑτητα
  • αΠΟ
  • πρωΙ