Αν τύχει να πέσει η κουρτίνα, ξέρω ότι φταίει το κουρτινόξυλο και όχι το παντζούρι.

It is an idiomatic way to say if it happens to..., if it so happens that..., or should ... happen.

So Αν τύχει να πέσει η κουρτίνα means something like:

• If the curtain happens to fall
• If the curtain should fall

The verb τυχαίνω normally means to happen or to chance to, and with να + subjunctive it often gives this sense of an accidental or unexpected event.

Because after να, Greek usually uses the subjunctive, and here πέσει is the aorist subjunctive form of πέφτω.

In this sentence, the fall is seen as one complete event:

• να πέσει = to fall / to end up falling once
• να πέφτει = to be falling / to keep falling / to fall repeatedly

So Αν τύχει να πέσει η κουρτίνα focuses on a single incident: the curtain falls.

Greek uses the definite article much more often than English does. So η κουρτίνα literally means the curtain, and that is completely normal even in places where English might sometimes omit an article in a more general statement.

Here it refers to a specific, understood curtain, so η κουρτίνα is exactly what you would expect.

Also, η κουρτίνα is the subject of πέσει.

Yes. Αν πέσει η κουρτίνα is perfectly natural and means if the curtain falls.

Adding τύχει να gives a slightly different nuance:

• Αν πέσει η κουρτίνα = plain condition
• Αν τύχει να πέσει η κουρτίνα = if the curtain happens to fall, with a sense of chance or accident

So the version with τύχει να sounds a bit more like if this should happen.

Φταίει comes from φταίω, which means:

• to be at fault
• to be to blame
• sometimes to be the cause of something

In this sentence, ξέρω ότι φταίει το κουρτινόξυλο means:

• I know the curtain rod is to blame
• I know it’s the curtain rod’s fault

So φταίει does not mean simple moral guilt only; it can also mean that something is the thing responsible for the problem.

Because with φταίει, the thing that is at fault is treated as the subject of the verb.

So:

• φταίει το κουρτινόξυλο = the curtain rod is to blame
• όχι το παντζούρι = not the shutter

Both nouns are in the nominative singular neuter:

• το κουρτινόξυλο
• το παντζούρι

This is different from English, where we often think in terms of blaming something as an object. Greek here is structured more like X is at fault.

Because ότι introduces a content clause: I know that...

So:

• ξέρω ότι φταίει... = I know that ... is to blame

By contrast, ξέρω να usually means I know how to do something:

• Ξέρω να διαβάζω = I know how to read

So ξέρω να φταίει would not work here.

Also, in everyday Greek, πως can often replace ότι in this kind of sentence:

• Ξέρω πως φταίει...

But ότι is perfectly standard.

Because the speaker is expressing a present certainty about a possible future event.

The meaning is:

• If that happens, I know now what the cause is
• My judgment is already clear

So:

• ξέρω = I know
• φταίει = is to blame

Greek often uses the present like this for something that is generally true or currently known, even when it applies to a future situation.

It is a compound noun:

• κουρτίνα = curtain
• ξύλο = wood

So literally it looks like curtain-wood, but in actual use it means curtain rod.

Even if the rod is not wooden, κουρτινόξυλο is still the normal word. This is very common in Greek: compounds may keep an older literal image, but the real meaning is fixed.

Παντζούρι means shutter, usually a window shutter, often the kind found outside the window.

So the contrast is:

• κουρτίνα = curtain
• κουρτινόξυλο = curtain rod
• παντζούρι = shutter

The sentence is saying that if the curtain falls, the problem is with the curtain rod, not with the shutter.

Greek word order is more flexible than English word order. Here the structure is natural because it builds toward the contrast:

• φταίει το κουρτινόξυλο
• και όχι το παντζούρι

That final και όχι... strongly highlights what is not to blame.

So the order feels like:

1. state who is responsible
2. reject the alternative

In English we might also naturally do something similar: I know it’s the curtain rod, not the shutter.

It is literally and not, but in sentences like this it works as a very natural contrast marker:

• X και όχι Y = X and not Y

It often sounds like:

• X rather than Y
• X, not Y

So here it is not simply adding another idea with and; it is correcting or contrasting:

• the curtain rod, not the shutter

It breaks down like this:

• Αν = if
• τύχει να πέσει η κουρτίνα = it happens that the curtain falls
• ξέρω = I know
• ότι = that
• φταίει το κουρτινόξυλο = the curtain rod is to blame
• και όχι το παντζούρι = and not the shutter

So the sentence contains:

1. a conditional clause: Αν τύχει να πέσει η κουρτίνα
2. a main clause: ξέρω ότι...
3. a content clause after ότι
4. a contrast at the end: και όχι...

That is a very common and useful Greek sentence pattern.