Learn Hardcore Greek: Καθώς περπατάμε στο μονοπάτι, περνάμε δίπλα από μια πηγή με καθαρό νερό. - As we walk on the path, we pass next to a spring with clean water.

Καθώς = as / while, focuses on simultaneity and often has a more descriptive, narrative feeling.
Όταν = when, more neutral; it can be used for single events or repeated situations.

In this sentence, you could say:

Όταν περπατάμε στο μονοπάτι, περνάμε δίπλα από μια πηγή…

It’s grammatically correct, but Καθώς sounds a bit more vivid and descriptive, like storytelling: you imagine the walking and passing by happening together in real time.

Why are both verbs (περπατάμε, περνάμε) in the present tense—is this like English continuous (are walking / are passing)?

Yes, this Greek present tense corresponds to English are walking / are passing here.

Greek doesn’t have a special continuous form; the present tense with the imperfective stem is used for:

general present actions (Περπατάμε κάθε μέρα = we walk every day)
actions happening right now / around now (Τώρα περπατάμε στο μονοπάτι = we are walking on the path)

So:

περπατάμε = we walk / we are walking
περνάμε = we pass / we are passing

Context (and sometimes adverbs) tells you whether it is “simple” or “continuous” in English. Here, the narrative Καθώς… clearly suggests a continuous, in-progress action: as we are walking… we are passing…

What is the difference between περπατάμε and περπατούμε? Are they both correct?

Both forms are correct; they are just stylistic variants of the 1st person plural present:

περπατάμε – more common in everyday modern speech and writing.
περπατούμε – a bit more formal or old-fashioned, but still used and understood.

They both mean we walk / we are walking. Many verbs in modern Greek have this pattern:

μιλάμε / μιλούμε (we speak)
τραγουδάμε / τραγουδούμε (we sing)

If in doubt, you can safely use the -άμε form in most everyday contexts.

What exactly is στο in στο μονοπάτι? Why not just σε το?

Στο is a contraction of σε + το:

σε = in / at / on (a general preposition of location)
το = the (neuter singular definite article, accusative case here)

Greek almost always contracts σε + το to στο in normal speech and writing:

σε το σπίτι → στο σπίτι (in/at the house)
σε το μονοπάτι → στο μονοπάτι (on the path)

So στο μονοπάτι literally is in/on the path or simply on the path in English. Saying σε το μονοπάτι feels unnatural and wrong in standard modern Greek.

Why is it δίπλα από? What is the difference between δίπλα, δίπλα σε, and δίπλα από?

Δίπλα means next to / beside, but in full phrases you usually combine it with a preposition:

δίπλα σε = next to (literally: beside at)
δίπλα από = next to / by (literally: beside from)

In everyday modern Greek, δίπλα σε and δίπλα από usually mean the same thing, and both are common:

περνάμε δίπλα σε μια πηγή
περνάμε δίπλα από μια πηγή

Both = we pass next to a spring.

You can sometimes use δίπλα alone when the context is clear:

Κάθισε δίπλα μου. = Sit next to me.

In your sentence, δίπλα από is just one natural choice; substituting δίπλα σε would not change the meaning.

Why do we say μια πηγή and not η πηγή? How does the Greek indefinite article work here?

Μια is the indefinite article for feminine singular nouns, equivalent to English a / an:

μια πηγή = a spring
η πηγή = the spring

We use μια πηγή when:

The spring has not been mentioned before.
It’s not a specific, uniquely identified spring for the listener.
We are introducing it into the story for the first time.

If the spring were already known in the context, you could use η πηγή:

Καθώς περπατάμε στο μονοπάτι, περνάμε δίπλα από την πηγή με το καθαρό νερό.
(As we are walking on the path, we pass by the spring with the clean water.)

So here μια πηγή presents it as some spring the listener is not expected to recognize beforehand.

What gender and case is πηγή, and why does μια have that form?

Πηγή (spring, fountain, source) is feminine in Greek.

Its main forms are:

η πηγή (nominative, subject: the spring)
της πηγής (genitive)
την πηγή (accusative, object: the spring)

After prepositions like από, Greek generally uses the accusative case, so we need the accusative:

από μια πηγή

Μια is the feminine singular form of the indefinite article in both nominative and accusative:

μια γυναίκα (a woman – subject)
βλέπω μια γυναίκα (I see a woman – object)
δίπλα από μια πηγή (next to a spring – object of the preposition)

So μια agrees with πηγή in gender (feminine) and number (singular), and the phrase is in the accusative because of από.

Why is it με καθαρό νερό and not something like με καθαρό το νερό or με καθαρού νερού?

Με here means with in the sense of containing / having:

μια πηγή με καθαρό νερό = a spring with clean water

Details:

νερό is neuter, singular.
καθαρό is the neuter singular form of the adjective, agreeing with νερό.
After με, we again use accusative, so καθαρό νερό is in the accusative.

Alternatives:

με καθαρού νερού is not correct; καθαρού is genitive and does not fit this structure.
με καθαρό το νερό is grammatically awkward and unnatural in this context. You could have με το νερό καθαρό, but that would sound like with the water being clean, a slightly different structure.

The simple and natural way to say with clean water is με καθαρό νερό.

How does adjective agreement work in μια πηγή με καθαρό νερό? Why καθαρό and not καθαρή?

In Greek, adjectives agree with the noun they describe in:

gender (masculine, feminine, neuter)
number (singular, plural)
case (nominative, accusative, etc.)

In the phrase:

μια πηγή – feminine singular
νερό – neuter singular
καθαρό – neuter singular form of καθαρός (clean)

Καθαρό does not describe πηγή, it describes νερό, so it must be neuter, like νερό:

μια καθαρή πηγή = a clean spring (adjective to πηγή, so καθαρή, feminine)
μια πηγή με καθαρό νερό = a spring with clean water (adjective to νερό, so καθαρό, neuter)

Changing καθαρό to καθαρή would wrongly make it agree with πηγή, which it does not directly modify in this sentence.

Can the word order be changed? For example, can I move καθώς περπατάμε or δίπλα από μια πηγή… to other positions?

Yes, Greek word order is relatively flexible, but some orders sound more natural than others.

Your sentence:

Καθώς περπατάμε στο μονοπάτι, περνάμε δίπλα από μια πηγή με καθαρό νερό.

Natural alternatives:

Περνάμε δίπλα από μια πηγή με καθαρό νερό καθώς περπατάμε στο μονοπάτι.
Καθώς περπατάμε στο μονοπάτι, δίπλα από μια πηγή με καθαρό νερό περνάμε. (a bit marked, but still acceptable)

What you usually don’t do is break the two main pieces too much in a confusing way, for example:

Καθώς περπατάμε, δίπλα από μια πηγή με καθαρό νερό περνάμε στο μονοπάτι.
This sounds awkward because στο μονοπάτι gets separated from περπατάμε and feels misplaced.

So yes, you can move phrases, but try to keep closely related parts together:

περπατάμε στο μονοπάτι
περνάμε δίπλα από μια πηγή με καθαρό νερό

Why is there a comma after μονοπάτι?

The comma separates a subordinate clause introduced by Καθώς from the main clause:

Subordinate clause: Καθώς περπατάμε στο μονοπάτι (As we are walking on the path)
Main clause: περνάμε δίπλα από μια πηγή με καθαρό νερό (we pass by a spring with clean water)

In Greek, just like in English, it is standard punctuation to put a comma after such an introductory clause:

Όταν φτάσουμε, θα σου τηλεφωνήσω.
Καθώς περπατάμε στο μονοπάτι, περνάμε δίπλα από μια πηγή…

If you reverse the order, the comma is usually not needed:

Περνάμε δίπλα από μια πηγή με καθαρό νερό καθώς περπατάμε στο μονοπάτι.