Αν είχα αφήσει το δίπλωμα στην τσέπη του παλιού παλτού, δεν θα είχα ξαναγυρίσει σπίτι μέσα στη βροχή.

This is a past counterfactual condition: it talks about something that did not happen in the past, and imagines what the result would have been.

Structure here:

• Αν είχα αφήσει... = If I had left...
• δεν θα είχα ξαναγυρίσει... = I would not have gone back...

So Greek is working very much like English:

• If I had done X, I would not have done Y.

This is one of the most common ways to express regret, hypothetical past situations, or what would have happened differently.

Both are compound forms built with έχω plus the main verb form.

1. είχα αφήσει

This is the past perfect / pluperfect idea:

• είχα = imperfect of έχω
• αφήσει = the form used in perfect tenses

So είχα αφήσει means I had left.

2. θα είχα ξαναγυρίσει

This is the conditional perfect idea:

• θα = marks conditional/future-style meaning
• είχα = had
• ξαναγυρίσει = gone back again / returned

So θα είχα ξαναγυρίσει means I would have gone back.

In short:

• είχα αφήσει = had left
• θα είχα ξαναγυρίσει = would have gone back

Because the sentence is talking about a hypothetical result in the past, not a present or future result.

Compare:

• θα γυρίσω = I will return / I would return
• θα είχα γυρίσει = I would have returned

So:

• θα + simple verb form often gives a future or less remote conditional meaning
• θα + είχα + verb form gives would have + past participle type meaning

Since this sentence is specifically about what would have happened in the past, θα είχα ξαναγυρίσει is exactly what you want.

In Greek, the negative particle δεν normally comes before the verb phrase, and with θα, the natural order is:

• δεν θα + verb

So:

• δεν θα πήγαινα
• δεν θα έλεγα
• δεν θα είχα ξαναγυρίσει

This is just the standard word order.
So δεν θα = would not.

ξαναγυρίσει comes from:

• ξανα- = again
• γυρίζω / γυρίσω / γυρίσει = return, go back, turn

So ξαναγυρίζω means:

• go back again
• return again
• sometimes simply go back

In this sentence, it suggests the speaker had to go back home again, probably because of the forgotten item.

So the ξανα- adds the idea of:

• repetition
• going back once more
• undoing your progress because of a mistake or forgotten thing

Greek often uses σπίτι by itself to mean home, especially after verbs of movement.

So:

• πάω σπίτι = I’m going home
• γυρίζω σπίτι = I’m returning home
• έμεινα σπίτι = I stayed home

This works a lot like English home, which also often appears without an article or preposition:

• go home
• come home
• stay home

You can also say στο σπίτι in many contexts, but that usually sounds more like to the house / at the house, while bare σπίτι often has the more idiomatic sense home.

μέσα στη βροχή literally means in the rain or more literally in the middle of the rain.

It adds a vivid sense of being out in it, surrounded by it, right in the rain.

Compare:

• στη βροχή = in the rain
• μέσα στη βροχή = right in the rain / out in the rain / in the middle of the rain

So μέσα makes the image stronger and more physical.

Also, στη is the contracted form of:

• σε + τη → στη

So:

• μέσα στη βροχή = μέσα σε τη βροχή (contracted in normal Greek)

This is about pronunciation and the way σε + article contracts.

• σε + την τσέπη → στην τσέπη
• σε + τη βροχή → στη βροχή

In modern everyday Greek:

• στην is often kept before vowel sounds and also commonly before some consonants
• στη is very common elsewhere

With τσέπη, speakers usually say στην τσέπη.

So this is normal idiomatic Greek, not something you need to over-analyze too much at first. The important thing is to learn the common contracted forms:

• στον
• στη(ν)
• στο

Because this phrase means of the old coat, and Greek expresses that with the genitive.

Break it down:

• το παλτό = the coat
• του παλτού = of the coat
• του παλιού παλτού = of the old coat

Both the adjective and the noun must match the grammatical case:

• παλιό παλτό = old coat
• παλιού παλτού = of an old coat / of the old coat

This whole phrase depends on η τσέπη:

• η τσέπη του παλιού παλτού = the pocket of the old coat

So Greek uses the genitive where English often uses of or an apostrophe structure.

δίπλωμα can mean different things depending on context:

• diploma / certificate
• driver’s license
• more generally, an official qualification document

In everyday Greek, very often το δίπλωμα means driver’s license.

So depending on the wider context, this sentence could sound like:

• If I had left my license in the pocket of the old coat...
• or If I had left the diploma/certificate...

A learner should be aware that δίπλωμα does not always map neatly to English diploma.

Not exactly. Modern Greek does not use the infinitive the way English does.

Instead, in compound tenses Greek uses a fixed verb form after έχω:

• έχω αφήσει = I have left
• είχα αφήσει = I had left
• θα είχα αφήσει = I would have left

So αφήσει here is the form used in these perfect-type constructions.

For an English speaker, the easiest way to think about it is:

• είχα αφήσει behaves like had left
• είχα γυρίσει behaves like had returned

You do not need to think of it as an English-style infinitive.

Because the sentence is about a single completed action imagined before another past event.

The idea is:

• first: possibly leaving the item in the coat pocket
• then: not needing to go back home

That is why Greek uses the perfect-style past form:

• Αν είχα αφήσει... = If I had left...

If you used άφηνα, that would suggest something more like an ongoing, habitual, or less appropriate past background action, and it would not fit this counterfactual structure well.

So είχα αφήσει is the natural choice because it refers to a completed action prior to the result.

Greek word order is more flexible than English, but this version is very natural and neutral.

Current order:

• Αν είχα αφήσει το δίπλωμα στην τσέπη του παλιού παλτού, δεν θα είχα ξαναγυρίσει σπίτι μέσα στη βροχή.

You could move some pieces for emphasis, for example:

• Αν είχα αφήσει στην τσέπη του παλιού παλτού το δίπλωμα...
• ...δεν θα είχα ξαναγυρίσει μέσα στη βροχή σπίτι.

But these may sound more marked, literary, or emphatic depending on context.

For a learner, the original order is a very good standard model:

• condition first
• result second
• object close to the verb
• location phrases placed naturally afterward

Yes, you could, and it would still be understandable.

But there is a slight difference in feel:

• γυρίσει σπίτι = go/return home (more idiomatic, very common)
• γυρίσει στο σπίτι = return to the house/home (a bit more explicit)

When Greek uses σπίτι without the article, it often works like English home, which is very natural after verbs of motion.

So in this sentence, ξαναγυρίσει σπίτι sounds especially natural and idiomatic.