Ό,τι κι αν λες, δεν αλλάζω την απόφασή μου.

Here Ό,τι means whatever or anything that.

The comma helps distinguish it from ότι, which usually means that.

• ό,τι = whatever / anything that
• ότι = that

So in this sentence, Ό,τι κι αν λες means something like whatever you say.

κι αν gives the phrase a concessive meaning: no matter what, even if, whatever.

So:

• Ό,τι λες would be more like what you say
• Ό,τι κι αν λες becomes whatever you say / no matter what you say

This is a very common Greek pattern:

• όπου κι αν πας = wherever you go
• όποιος κι αν είναι = whoever he/she may be
• ό,τι κι αν κάνεις = whatever you do

κι is just the shortened form of και.

Before a word starting with a vowel, Greek very often uses κι instead of και, especially in natural everyday speech and writing.

So these are equivalent:

• ό,τι κι αν λες
• ό,τι και αν λες

The version with κι sounds very normal and smooth.

Because this pattern uses αν, not να.

In expressions like ό,τι κι αν λες, αν is part of a fixed structure meaning whatever / no matter what.

So Greek says:

• ό,τι κι αν λες
• όποιος κι αν έρθει
• όπου κι αν πάμε

You should learn ό,τι κι αν... as a chunk.

A learner may also hear ό,τι και να λες, which is also very common in Modern Greek, especially in speech. It has basically the same meaning.

Yes, Greek could also use πεις:

• Ό,τι κι αν λες
• Ό,τι κι αν πεις

Both are natural, but they are not exactly the same in aspect.

• λες gives a more ongoing / general / repeated sense: whatever you say
• πεις gives a more single / complete / specific sense: whatever you may say / whatever you end up saying

In many contexts, the difference is small, but Greek often uses aspect this way.

Because Greek often uses the present tense to express a firm present stance that also clearly applies to the future.

So δεν αλλάζω την απόφασή μου can mean:

• I am not changing my decision
• I won’t change my decision

The Greek present here sounds strong and definite.

If you wanted a more explicitly future form, you could say:

• Δεν θα αλλάξω την απόφασή μου

But the present tense in your sentence is very idiomatic and forceful.

Because Ό,τι κι αν λες is an introductory subordinate clause.

Greek normally separates this kind of fronted clause from the main clause with a comma:

• Ό,τι κι αν λες, δεν αλλάζω την απόφασή μου.

It works much like English punctuation in:

• Whatever you say, I’m not changing my decision.

Greek usually keeps the definite article even when a noun has a possessive pronoun.

So Greek normally says:

• η μητέρα μου = my mother
• το σπίτι μου = my house
• την απόφασή μου = my decision

In English, we usually do not use the with my, but in Greek this is normal and expected.

Because μου is an enclitic word, and Greek spelling often adds an extra accent to the preceding word to show the correct stress.

The base word is:

• απόφαση

But when followed by enclitic μου, it becomes:

• απόφασή μου

This helps preserve the natural pronunciation of the phrase.

You will see this with many nouns and adjectives followed by μου, σου, του, της, etc.

It is in the accusative case because it is the direct object of αλλάζω.

• αλλάζω κάτι = I change something

So:

• δεν αλλάζω την απόφασή μου = I do not change my decision

The article την also shows that the noun phrase is feminine singular accusative.

Yes. Greek word order is flexible.

You could also say:

• Δεν αλλάζω την απόφασή μου, ό,τι κι αν λες.

This has the same basic meaning, but the emphasis changes slightly.

• Starting with Ό,τι κι αν λες puts emphasis on whatever you say
• Starting with Δεν αλλάζω την απόφασή μου puts emphasis on I am not changing my decision

Both are natural.

Yes, in everyday Modern Greek they are very close in meaning.

• ό,τι κι αν λες
• ό,τι και να λες

Both mean whatever you say / no matter what you say.

Very roughly:

• κι αν can sound a bit more compact or slightly more formal/literary in some contexts
• και να is extremely common in speech

A learner should understand both, because both are widely used.