Οι ρίζες σαπίζουν από το πολύ νερό.

Οι ρίζες means the roots.

• οι = the for plural nouns
• ρίζες = roots

Here, ρίζες is a feminine plural noun, and it is the subject of the sentence, so it appears in the nominative plural.

Singular forms:

• η ρίζα = the root
• οι ρίζες = the roots

Because the subject is plural: οι ρίζες = the roots.

So the verb must also be plural:

• η ρίζα σαπίζει = the root rots / is rotting
• οι ρίζες σαπίζουν = the roots rot / are rotting

So σαπίζουν is the 3rd person plural form.

σαπίζουν is the present tense, 3rd person plural, from the verb σαπίζω = to rot / to make rot.

In this sentence it is used intransitively, meaning the roots themselves are rotting:

• σαπίζω = I rot / I cause to rot
• σαπίζει = it rots / is rotting
• σαπίζουν = they rot / are rotting

In natural English, this can be translated as either:

• rot
• are rotting

depending on context.

Greek often uses a simple present tense where English may choose either:

• The roots rot
• The roots are rotting

So σαπίζουν can cover both ideas. Greek does not need a separate auxiliary verb like are here.

Also, even though English often uses are rotting, Greek does not need a passive form. σαπίζουν is a normal active verb used in an intransitive way.

Here από means from, but in this sentence it expresses cause:

• από το πολύ νερό = from too much water / because of too much water

So it is not about physical movement. It is explaining why the roots are rotting.

Greek often uses από this way to mean:

• because of
• due to
• from

Both can exist, but από το πολύ νερό is a very natural Greek way to say because of the too much water or more naturally because of too much water.

The article το makes πολύ νερό into a more complete noun phrase:

• το πολύ νερό = the excess water / the large amount of water

This structure is common in Greek when talking about a condition or cause.

So:

• από πολύ νερό = possible, more general
• από το πολύ νερό = very natural, often slightly more specific or idiomatic in context

Literally, πολύ means much / a lot.

But in this sentence, because the roots are rotting, the natural meaning is too much water. So English usually translates it that way.

So:

• literal Greek structure: from the much water
• natural English meaning: because of too much water

Greek does not always need a separate word exactly matching English too in this kind of context.

Because water is normally treated as an uncountable noun in Greek, just as in English.

So Greek says:

• νερό = water

not a plural form meaning waters in this everyday sense.

When talking about quantity, Greek still keeps it singular:

• πολύ νερό = a lot of water
• λίγο νερό = a little water

The given order is the most neutral and natural:

• Οι ρίζες σαπίζουν από το πολύ νερό.

Greek word order is fairly flexible, so other orders are possible for emphasis, such as putting the cause first. But the basic order here is clear and standard:

• subject: Οι ρίζες
• verb: σαπίζουν
• cause phrase: από το πολύ νερό

For a learner, the given word order is the best one to use first.

A simple pronunciation guide is:

i RÍ-zes sa-PÍ-zun a-PÓ to po-LÍ ne-RÓ

A few stress points:

• ρίζες → stress on the first syllable: RÍ-zes
• σαπίζουν → stress on πί: sa-PÍ-zun
• πολύ → stress on the second syllable: po-LÍ
• νερό → stress on the last syllable: ne-RÓ

Also note:

• οι is pronounced like ee
• από is pronounced a-PO

Yes. That would be a very natural English rendering too.

The Greek sentence allows slightly different English translations depending on style:

• The roots are rotting from too much water.
• The roots rot because of too much water.
• The roots are decaying from all the water.

The Greek grammar stays the same; only the English phrasing changes.