Learn Hardcore Greek: Η απάντηση που μου έδωσες ήταν πολύ σημαντική. - The answer that you gave me was very important.

It introduces a restrictive relative clause modifying η απάντηση. Here που means “that/which/who,” and it doesn’t change form (it’s invariable). Don’t confuse it with:

ότι/πως = “that” introducing a content clause (e.g., “I know that …”)
πού (with an accent) = “where” as a question word

Why is μου used instead of με or εμένα?

μου is the weak genitive pronoun meaning “to me/for me” (indirect object). That fits verbs like “give.”
με is the weak accusative pronoun meaning “me” as a direct object.
εμένα is the strong (stressed) form “me,” used for emphasis or after prepositions (e.g., σε εμένα “to me”).
In this sentence, the recipient is “me,” so μου is the right choice.

Why does μου come before έδωσες (and not after it)?

Weak object pronouns in Greek usually come before the verb (proclisis) in statements and in subordinate clauses (and especially after words like που, δεν, να, θα). They come after the verb only with affirmative imperatives and a few other forms.
So: μου έδωσες (you gave me), but imperative: δώσε μου (give me).

What tense/aspect is έδωσες?

It’s the aorist (simple past), perfective aspect, 2nd person singular of δίνω “to give.”

Present: δίνω / δίνεις / δίνει …
Aorist: έδωσα / έδωσες / έδωσε … The aorist presents the action as a single, completed event (“you gave”).

Could I use a present-tense form like δίνεις here?

You could, but it would change the meaning:

που μου δίνεις = “that you (are) give/giving me” (present/habitual)
που μου έδινες = “that you were giving me/used to give me” (imperfect, ongoing in the past)
που μου έδωσες = “that you gave me” (aorist, one completed past action)
In this sentence, the completed-past sense is intended, so έδωσες fits best.

What is ήταν exactly?

It’s the past of είμαι (“to be”)—the imperfect tense—3rd person singular: “was.” Colloquially you may also see ήτανε. (For 3rd plural, ήταν/ήτανε is also used.)

Why is σημαντική (feminine) used instead of σημαντικό (neuter) or σημαντικός (masculine)?

Adjectives agree with the noun they describe. απάντηση is feminine (η απάντηση), so the predicate adjective is feminine: σημαντική.

Masculine: σημαντικός
Feminine: σημαντική
Neuter: σημαντικό

Is πολύ inflected here? Why not πολλή?

Before an adjective, πολύ means “very” and is an invariable adverb (it never changes): πολύ σημαντική = “very important.”
πολύς / πολλή / πολύ is the inflected adjective meaning “much/many” (e.g., πολλές απαντήσεις = “many answers,” πολλή σημασία = “much importance”). Don’t use that form with an adjective meaning “very.”

Why is there a definite article Η?

Greek uses the definite article more broadly than English. With a defining relative clause (που μου έδωσες) it points to a specific answer already identifiable in context.

Η απάντηση … = a particular answer.
Μια απάντηση … = “an answer …,” implying one among several.

Do I need a comma before που?

No comma for a restrictive relative clause (the normal case here): Η απάντηση που …
You can use commas for a non-restrictive, parenthetical clause (extra information), but that changes the nuance and is less typical in Greek with που.

Can I use ο οποίος instead of που?

Yes, but it’s more formal and it inflects for gender/number/case. Because the relative pronoun is the direct object of έδωσες, you need the accusative:

Η απάντηση την οποία μου έδωσες ήταν πολύ σημαντική.
Formally, ο οποίος also lets you use prepositions (e.g., στην οποία, με την οποία), which που cannot take.

Is που μου την έδωσες acceptable (with την)?

You’ll hear clitic “doubling” in speech (e.g., που μου την έδωσες), but standard written Greek normally omits that extra object pronoun here. The concise, standard form is που μου έδωσες.

What is the direct object of έδωσες in the relative clause?

It’s understood to be the antecedent η απάντηση. The relative που links the clause to that noun, so you don’t repeat the object inside the clause. μου is the indirect object (“to me”).

How do you pronounce the whole sentence?

Approximate: “ee a-PAN-dee-see poo moo EH-tho-ses EE-tan po-LEE see-man-dee-KEE.”
A simple transliteration: I apándisi pu mu édoses ítan polí simantikí.
Notes:

δ = voiced “th” (as in “this”)
Final -η in σημαντική sounds like “ee.”

Why is it Η (no accent) at the beginning? Isn’t Ή also a word?

Η (no accent) is the feminine nominative article “the.”
Ή (with an accent) is the conjunction “or.”
At the start of a sentence, the accent is written if it’s the conjunction (Ή), but the article Η has no accent.

What’s the difference between που and πού?

που (no accent) = relative marker “that/which/who.”
πού (accented) = the question word “where.”
For relative “where,” written Greek prefers όπου; colloquially you’ll also hear που used with place nouns.

How do I say this with polite/plural “you”?

Change the verb to 2nd person plural:

Η απάντηση που μου δώσατε ήταν πολύ σημαντική.

How do I say “the answer you have given me” (present perfect-like)?

Use έχω δώσει:

Η απάντηση που μου έχεις δώσει … (singular “you”)
Η απάντηση που μου έχετε δώσει … (polite/plural)
This ties the past act to the present more than the simple aorist.

How do I flip it to “the answer I gave you was very important”?

Swap the pronoun:

Η απάντηση που σου έδωσα ήταν πολύ σημαντική. (to you, singular)
Η απάντηση που σας έδωσα ήταν πολύ σημαντική. (to you, plural/polite)

Can πολύ come after the adjective (e.g., “σημαντική πολύ”)?

Normally, πολύ comes before the adjective: πολύ σημαντική. Post-position (σημαντική πολύ) can appear for stylistic emphasis in speech, but it’s marked and less common.