Learn Hardcore Greek: Το δάσος αυτό κινδυνεύει όταν έχουμε φωτιά το καλοκαίρι. - This forest is in danger when we have a fire in the summer.

αυτό το δάσος is the more neutral, very frequent everyday form.
το δάσος αυτό can sound a bit more contrastive or specific, like this particular forest (as opposed to others), especially in written or slightly more formal language.

In practice, in a sentence like this, they are almost interchangeable.

What exactly does κινδυνεύει mean, and why use this verb instead of something like είναι σε κίνδυνο?

Κινδυνεύει is the 3rd person singular of the verb κινδυνεύω, which means to be in danger, to be at risk, to be threatened.

το δάσος = the forest (3rd person singular, neuter)
κινδυνεύει = is in danger / is at risk

So Το δάσος αυτό κινδυνεύει literally means This forest is in danger.

You could also say:

Το δάσος αυτό είναι σε κίνδυνο
Το δάσος αυτό βρίσκεται σε κίνδυνο

These are correct too, but κινδυνεύει is shorter and very natural here.

The verb κινδυνεύω is used both literally and figuratively:

Το παιδί κινδυνεύει. = The child is in danger.
Το σπίτι κινδυνεύει να καεί. = The house is in danger of burning down.

Why is the present tense used (κινδυνεύει, έχουμε) even though this sounds like a general or future situation?

In Greek, the present tense is normally used for:

General truths / repeated situations
- Το δάσος αυτό κινδυνεύει όταν έχουμε φωτιά το καλοκαίρι.
  = This forest is in danger whenever there is fire in the summer.
Future events in time clauses with words like όταν (when), μόλις (as soon as), etc.
Greek usually does not use θα in the όταν-clause:

Natural: Θα κινδυνεύει το δάσος όταν έχουμε φωτιά.
Not natural: Θα κινδυνεύει το δάσος όταν θα έχουμε φωτιά.

So using the present κινδυνεύει, έχουμε here is exactly what Greek normally does for a general, repeated situation.

Why does the sentence use έχουμε φωτιά (we have fire) instead of something like υπάρχει φωτιά (there is fire)?

All of these are grammatically possible, but they differ slightly in nuance:

όταν έχουμε φωτιά
Literally when we have a fire. It implies people (we, the community, the area) are experiencing a fire. It can sound a bit more involved or personal.
όταν υπάρχει φωτιά
Literally when there is a fire. More impersonal and neutral, just stating that a fire exists.
όταν έχει φωτιά
Colloquial impersonal έχει meaning there is (similar to έχει ζέστη = it is hot). This is also natural in spoken Greek in some regions.

In your sentence, όταν έχουμε φωτιά is fine and quite natural; όταν υπάρχει φωτιά would also be perfectly acceptable.

Why is there no article before φωτιά? Why not μια φωτιά or τη φωτιά?

Here φωτιά is used without an article to talk about fire in general / any fire in this kind of situation.

όταν έχουμε φωτιά ≈ whenever there is fire / whenever a fire breaks out

Possible alternatives and their nuance:

όταν έχουμε μια φωτιά = when we have a (single, specific) fire
Slightly more like referring to one particular incident.
όταν έχουμε τη φωτιά would normally refer to the fire already mentioned/known, so it would need a clear previous reference.

In many expressions with έχω, Greek often omits the article for indefinite, general situations:

Έχω δουλειά. = I have work (to do).
Έχω πονοκέφαλο. = I have a headache.
Έχουμε φωτιά. = We have a fire (there is a fire here).

So the lack of article makes the idea more general and indefinite.

Could we say φωτιές (plural) instead of φωτιά (singular)? Would that change the meaning?

Yes, you could say:

Το δάσος αυτό κινδυνεύει όταν έχουμε φωτιές το καλοκαίρι.

The difference:

φωτιά (singular) here refers to the phenomenon of fire in general. It can still cover multiple actual fires over time; it is generic.
φωτιές (plural) focuses more on there being many separate fires, e.g. multiple incidents each summer.

Both are understandable. The singular is slightly more abstract/general; the plural makes you think more concretely about many different fires.

Why do we say το καλοκαίρι with the article? How does that mean in the summer if there is no word like in?

In Greek, seasons and many time expressions often use the accusative with the definite article to mean in / during that time:

το καλοκαίρι = in the summer
τον χειμώνα = in the winter
την άνοιξη = in the spring
το φθινόπωρο = in the autumn

There is no separate word for in here; the article + accusative time expression itself functions adverbially and means in/during X.

So:

όταν έχουμε φωτιά το καλοκαίρι
= when we have fire in the summer

You can sometimes drop the article and just say καλοκαίρι in an adverbial way (especially in short phrases like Πάω Ελλάδα καλοκαίρι), but το καλοκαίρι is the most standard and natural form here.

Why is the subject pronoun εμείς (we) not used before έχουμε?

Greek is a pro‑drop language: subject pronouns (εγώ, εσύ, αυτός... εμείς, εσείς, αυτοί...) are often left out because the verb ending already shows the person and number.

έχουμε already tells you we are the subject.

You would normally say:

Όταν έχουμε φωτιά το καλοκαίρι… = When we have fire in the summer…

You only add εμείς for emphasis or contrast:

Όταν εμείς έχουμε φωτιά, κανείς δεν βοηθάει.
= When we have a fire, nobody helps. (contrast: maybe others get help)

So in the neutral sentence, leaving out εμείς is the natural choice.

Could we use αν instead of όταν? What is the difference between όταν έχουμε φωτιά and αν έχουμε φωτιά?

Both can work, but they are not identical:

όταν έχουμε φωτιά το καλοκαίρι
Literally when(ever) we have fire in the summer.
Emphasises time and usually implies that this does in fact happen at some point (or regularly).
αν έχουμε φωτιά το καλοκαίρι
Literally if we have fire in the summer.
Emphasises condition / possibility. It sounds a bit more hypothetical.

So:

Το δάσος αυτό κινδυνεύει όταν έχουμε φωτιά το καλοκαίρι.
= This forest is in danger whenever there is fire in the summer. (a regular, expected situation)
Το δάσος αυτό κινδυνεύει αν έχουμε φωτιά το καλοκαίρι.
= This forest is in danger if we have fire in the summer. (more conditional, “in case that happens”)

How flexible is the word order in this sentence? Can we move το καλοκαίρι or the όταν‑clause?

Greek word order is quite flexible, so you can rearrange parts mainly to change emphasis, without changing the basic meaning. For example:

Αυτό το δάσος κινδυνεύει όταν έχουμε φωτιά το καλοκαίρι.
Το δάσος αυτό κινδυνεύει το καλοκαίρι, όταν έχουμε φωτιά.
Όταν έχουμε φωτιά το καλοκαίρι, το δάσος αυτό κινδυνεύει.

All of these are natural. Differences are mostly in what you highlight:

Starting with Όταν έχουμε φωτιά… highlights the condition first.
Moving το καλοκαίρι earlier can put more stress on the season.

What you cannot do is break noun phrases apart (for example, you cannot separate το from δάσος or καλοκαίρι), but moving whole chunks like το καλοκαίρι or the entire όταν‑clause is normal.