Learn Hardcore Greek: Θα περνάμε πολύ καλά αν έρθεις κι εσύ. - We will have a very good time if you come too.

Θα περνάμε (future continuous/imperfective) suggests ongoing or overall enjoyable time during a period: we’ll be having fun throughout.
Θα περάσουμε (future simple/perfective) presents the event as a single whole: we’ll have a great time (as an outcome). Both are correct; pick based on whether you imagine a stretch of time (trip, weekend) vs. a single occasion.

Why is έρθεις used and not έρχεσαι after αν?

After αν, Greek typically uses the perfective non-past (subjunctive-like) form for a single future event: αν έρθεις.
αν έρχεσαι would refer to a repeated or ongoing action (if you come regularly/if you’re in the habit of coming).

Why is there no θα in the αν-clause?

In standard Greek you don’t use θα after αν to talk about the future. The pattern is:

αν
- perfective non-past (e.g., αν έρθεις)
main clause with θα (e.g., θα περνάμε) Forms like αν θα έρθεις are non-standard in most contexts.

What does κι mean in κι εσύ, and why not και?

κι is the same word as και (and/also). It’s the shortened form used before a vowel for euphony. Here it means too/as well.
Both κι εσύ and και εσύ are correct; κι sounds smoother before εσύ. No apostrophe is needed.

Is εσύ necessary here?

No. The verb έρθεις already shows the subject.

αν έρθεις = if you come (neutral)
αν έρθεις κι εσύ = if you come too (adds emphasis/contrast)
You can also say αν κι εσύ έρθεις to emphasize you.

What does the expression περνάω καλά mean?

Literally, to pass/spend time well; idiomatically, to have a good time. Very common.
Variants/intensifiers: περνάω πολύ καλά, περνάω ωραία, περνάω τέλεια.
Colloquially you’ll also hear: Θα το περάσουμε πολύ καλά (same meaning).

Would Θα περάσουμε πολύ καλά αν έρθεις κι εσύ be more natural?

Yes, if you mean one specific occasion. Θα περνάμε highlights the enjoyment as ongoing during the time you’re together. Both are fine.

Can I start with the if-clause, and do I need a comma?

Yes: Αν έρθεις κι εσύ, θα περνάμε πολύ καλά.
When the αν-clause comes first, use a comma. When it comes after the main clause, a comma is usually not used.

Why πολύ καλά and not πολλή καλή or πολλά καλά?

καλά here is an adverb (well), so it’s modified by the invariable adverb πολύ (very).
πολλή/πολλά/πολλοί are adjective forms meaning many/much and don’t fit this adverbial use.

Is εάν different from αν?

Same meaning. εάν is a bit more formal or careful; αν is the everyday form. Both are acceptable: Εάν έρθεις κι εσύ… / Αν έρθεις κι εσύ…

How do I address someone politely or a group?

Use second-person plural: Θα περνάμε πολύ καλά αν έρθετε κι εσείς.
Here έρθετε is the 2nd person plural, and εσείς is the polite/plural pronoun.

How do I make the sentence negative?

Δε(ν) θα περάσουμε/περνάμε πολύ καλά αν δεν έρθεις (κι εσύ).

Negate the main clause with δε(ν) before θα.
After αν, use δεν (standard Greek uses δεν with αν): αν δεν έρθεις.

Is this a real future condition or a hypothetical/unreal one?

It’s a real, open future condition: αν έρθεις … θα ….
For a counterfactual you’d use past forms: Θα περνούσαμε πολύ καλά αν ερχόσουν (we would have a great time if you came, implying you probably won’t).

What exactly is the form έρθεις, and how is it pronounced?

It’s the 2nd person singular perfective non-past of έρχομαι used in subjunctive-like environments: (να) έρθω, έρθεις, έρθει, έρθουμε, έρθετε, έρθουν(ε). After αν, you use the same form without να: αν έρθεις.
Pronunciation tips: Θ as in English thin; stresses: περ-νά-με, πο-λύ, κα-λά, έρ-θεις; κι εσύ flows together smoothly.