Όταν βλέπω αστραπή, κλείνω αμέσως το παράθυρο.

Here it most naturally means whenever.

Because both clauses use the present tense, the sentence describes a habit or repeated reaction:

• Όταν βλέπω αστραπή, κλείνω αμέσως το παράθυρο.
• Whenever I see lightning, I immediately close the window.

If you were talking about one future occasion, Greek would normally use a different verb form:

• Όταν δω αστραπή, θα κλείσω αμέσως το παράθυρο.
• When I see lightning, I will immediately close the window.

Because Greek often uses the present tense for habitual or general actions, just like English can use the simple present for routines.

So this sentence does not mean that the speaker is necessarily seeing lightning right now. It means this is what the speaker usually does in that situation.

Also, both forms are 1st person singular:

• βλέπω = I see
• κλείνω = I close

The ending helps show the subject, so Greek does not need a separate word for I.

Greek is a pro-drop language, which means the subject pronoun is often omitted when the verb ending already makes it clear.

Here:

• βλέπω already means I see
• κλείνω already means I close

You could add εγώ if you wanted emphasis or contrast:

• Όταν βλέπω αστραπή, εγώ κλείνω αμέσως το παράθυρο.

But in a normal neutral sentence, leaving εγώ out is more natural.

This is mainly about aspect and type of meaning.

• βλέπω is the imperfective/present form, used for ongoing, repeated, or habitual actions.
• δω is the perfective form, often used for a single future event after words like όταν.

So:

• Όταν βλέπω αστραπή... = Whenever I see lightning...
  habitual, repeated

• Όταν δω αστραπή... = When I see lightning...
  one future occurrence

A native English speaker often has to learn that Greek makes this distinction more clearly than English does.

Because Greek can leave out the article when a noun is being used in a general or indefinite way.

Here αστραπή means something like:

• lightning
• a flash of lightning

Without the article, it sounds natural and general.

Compare:

• βλέπω αστραπή = I see lightning / a flash of lightning
• βλέπω μια αστραπή = I see a flash of lightning
  a bit more concrete, explicitly one
• βλέπω την αστραπή = I see the lightning / the flash
  a specific one, already known from context

Greek uses the definite article very often, sometimes more often than English.

Here το παράθυρο most naturally means the window in the situation being talked about — the relevant, specific window the speaker would close.

In English, sometimes we might say the window or even my window depending on context. Greek often prefers the article with concrete nouns like this.

So:

• κλείνω το παράθυρο = I close the window

Without the article, κλείνω παράθυρο, the sentence would sound much less natural in this context.

Both are in the accusative case, because they are the direct objects of the verbs.

• βλέπω αστραπή → αστραπή is what I see
• κλείνω το παράθυρο → το παράθυρο is what I close

A useful detail:

• αστραπή is feminine, and its singular nominative and accusative look the same
• παράθυρο is neuter, and its singular nominative and accusative also look the same

So even though the forms do not change visibly here, grammatically they are accusatives.

Αμέσως means immediately, right away, or at once.

In this sentence it comes between the verb and the object:

• κλείνω αμέσως το παράθυρο

That word order is very natural. But Greek word order is fairly flexible, so you could also hear:

• κλείνω το παράθυρο αμέσως
• αμέσως κλείνω το παράθυρο

The basic meaning stays the same, though the emphasis may shift slightly.

Because Όταν βλέπω αστραπή is a subordinate time clause placed before the main clause.

So the structure is:

• Όταν βλέπω αστραπή, = when/whenever I see lightning
• κλείνω αμέσως το παράθυρο. = I immediately close the window

Using a comma here is standard and helps separate the two parts clearly.

Yes, you could.

There is a small difference in feel:

• Όταν βλέπω αστραπή = a general, natural way to say whenever I see lightning
• Όταν βλέπω μια αστραπή = whenever I see a flash of lightning
  slightly more concrete and countable

So both are possible, but the version without μια sounds a bit more idiomatic as a general statement.

Not exactly.

• αστραπή = lightning, flash of lightning
• κεραυνός = thunderbolt, lightning strike

In many situations they are related, but they are not always interchangeable.

In this sentence, αστραπή is the more natural choice because the speaker is reacting to seeing lightning.