Αν ξεχάσουμε το σπρέι στο σπίτι, τα κουνούπια μας ενοχλούν όλη τη βραδιά.

Αν means if and introduces a conditional clause.

So the sentence has two parts:

• Αν ξεχάσουμε το σπρέι στο σπίτι = If we forget the spray at home
• τα κουνούπια μας ενοχλούν όλη τη βραδιά = the mosquitoes bother us all evening

In Modern Greek, αν is the normal word for real/open conditions like if that happens.

Ξεχάσουμε is the perfective non-past form of ξεχνάω / ξεχνώ. After αν, Greek often uses this form when the action is seen as a single complete event.

Here, forgetting the spray is one completed action:

• we either forget it or we do not.

So:

• αν ξεχάσουμε = if we forget
• αν ξεχνάμε would sound more like if we are forgetting or if we tend to forget, which is not the most natural idea here.

This is a very common Greek pattern:

• αν + perfective non-past for a single event
• αν + imperfective non-past for repeated, ongoing, or habitual action

Yes, ξεχάσουμε is the same form used in the subjunctive system, but after αν Greek does not use να.

So you get:

• να ξεχάσουμε = that we forget / for us to forget
• αν ξεχάσουμε = if we forget

A useful way to think about it is:

• να and αν both introduce environments where Greek uses these non-past verb forms
• but they do different jobs:
  • να introduces subjunctive-type clauses
  • αν introduces conditional clauses

Greek often leaves out subject pronouns when they are not needed, because the verb ending already shows who the subject is.

Here:

• ξεχάσουμε = we forget
• the ending -ουμε tells you it is we

So Greek does not need εμείς unless you want emphasis.

Compare:

• Αν ξεχάσουμε... = If we forget...
• Αν εμείς ξεχάσουμε... = If we forget... with extra emphasis, perhaps contrasting us with someone else

Σπρέι is a neuter noun in Modern Greek, and here it is in the accusative singular as the direct object of ξεχάσουμε.

• το = neuter singular article
• σπρέι = spray

So:

• το σπρέι = the spray

A useful point: σπρέι is a loanword and is generally indeclinable, so the noun itself does not change form. The article tells you its gender, number, and case.

Στο σπίτι is a contraction of:

• σε + το σπίτι → στο σπίτι

It can mean at home, in the house, or sometimes to the house/home, depending on context.

Here, with forget the spray at home, it means:

• at home
• or more literally, in the house

So:

• στο σπίτι = at home / in the house

Greek uses the definite article much more often than English.

So where English says:

• at home

Greek naturally says:

• στο σπίτι
  literally, in/at the house

This is normal Greek usage, not something especially emphatic. English and Greek simply handle articles differently here.

Greek often uses the definite article with nouns in a generic sense, where English may use no article.

So:

• τα κουνούπια literally = the mosquitoes
• but in natural English it often just means mosquitoes in general

This is very common in Greek. The article does not always mean a specific, previously mentioned set in the same way it often does in English.

Μας is the weak object pronoun meaning us here, and weak object pronouns normally come before the finite verb in Modern Greek.

So:

• τα κουνούπια μας ενοχλούν = the mosquitoes bother us

Breakdown:

• τα κουνούπια = the subject
• μας = us
• ενοχλούν = bother

This word order is very normal:

• subject + object pronoun + verb

Also note that μας can mean either us or our in other contexts, but here it clearly means us.

The present tense here gives the idea of a usual result or a general truth:

• If we forget the spray at home, mosquitoes bother us all evening.

Greek often uses the present in this kind of conditional sentence when describing what generally happens.

So:

• αν ξεχάσουμε..., τα κουνούπια μας ενοχλούν...
  means something like whenever that happens, this is the result

If you wanted to focus on a specific future occasion, Greek might use a future form in the main clause instead, depending on the context.

Όλη τη βραδιά means all evening / the whole evening.

Grammatically:

• όλη = whole / all
• τη βραδιά = the evening in the accusative

The whole phrase is an accusative of duration, meaning it tells you how long something happens.

So:

• μας ενοχλούν όλη τη βραδιά = they bother us all evening

Both relate to the same article την.

In Modern Greek, the final -ν is often dropped before certain consonants, especially in everyday writing and speech. So:

• την βραδιά → often written/spoken as τη βραδιά

That is completely normal.

So:

• όλη τη βραδιά is just the usual reduced form
• it still means exactly the same as όλη την βραδιά

Both relate to evening/night, but they are not always used in exactly the same way.

Very roughly:

• βράδυ is the more basic, neutral word for evening
• βραδιά often refers more to the span of the evening, or to the evening as an occasion/period

That is why όλη τη βραδιά sounds very natural for all evening.

You could compare:

• όλο το βράδυ = all night / all evening, depending on context
• όλη τη βραδιά = the whole evening

In many situations they overlap, but βραδιά often feels a little more like the course of the evening.

Yes. Greek word order is more flexible than English because case, articles, and verb endings help show the relationships between words.

The sentence as given is very natural:

• Αν ξεχάσουμε το σπρέι στο σπίτι, τα κουνούπια μας ενοχλούν όλη τη βραδιά.

But other orders are possible for emphasis, for example:

• Τα κουνούπια μας ενοχλούν όλη τη βραδιά, αν ξεχάσουμε το σπρέι στο σπίτι.

That said, the original order is the clearest and most neutral:

1. condition first
2. result second