Learn Hardcore Greek: Είμαι εκατό τοις εκατό σίγουρος για την απόφασή μου σήμερα. - I am one hundred percent sure about my decision today.

Τοις is actually the old dative plural of the definite article ο / η / το. Historically, εκατό τοις εκατό literally means “one hundred per hundred” (like “100 per 100”).

In modern Greek:

The dative case is almost gone in normal grammar.
But it survives in a few fixed expressions, like:
- εκατό τοις εκατό = 100%
- εν τω μεταξύ (in the meantime)
- τοις μετρητοίς (in cash, literally “with cash payments”)

Today, people don’t feel τοις as a normal word you decline; they just know the whole phrase εκατό τοις εκατό as a set expression for 100%.

Could I just say είμαι 100% σίγουρος instead of είμαι εκατό τοις εκατό σίγουρος?

Yes. In everyday speech:

Είμαι 100% σίγουρος.
Είμαι εκατό τοις εκατό σίγουρος.

Both are completely natural and mean the same thing. You will often see 100% written with numerals in informal writing, advertising, messages, etc.

Slight nuance:

Εκατό τοις εκατό sounds a bit more spoken / emphatic.
100% is more visual / modern, like in English.

Why do we use για with σίγουρος? Could I say something else instead of για?

The adjective σίγουρος commonly takes the preposition για to introduce what you are sure about:

Είμαι σίγουρος για την απόφασή μου.
I am sure about my decision.

Other common patterns:

σίγουρος για κάτι = sure about something
σίγουρος ότι… = sure that…
- Είμαι σίγουρος ότι έχεις δίκιο.

You wouldn’t normally replace για with another preposition here. You could rephrase with a different structure:

Είμαι απόλυτα σίγουρος με την απόφασή μου.
This sounds less standard and more like “I am completely comfortable with my decision.”

But the natural and standard way for “sure about X” is σίγουρος για X.

What case is την απόφασή μου, and why do we have την?

Την απόφασή μου is in the accusative singular, feminine.

Breakdown:

η απόφαση (the decision) – nominative
την απόφαση (the decision) – accusative (basic form after many prepositions)
την απόφασή μου – accusative + enclitic μου (my)

We use για + accusative:

για
- την απόφασή μου = about my decision

The article την:

Shows that απόφαση is feminine, singular, accusative.
Is normally required; dropping it here (για απόφασή μου) would sound incomplete or poetic.

So για την απόφασή μου is the normal, grammatical form.

Why is there an accent on the last syllable in απόφασή μου and not in απόφαση?

The base word is:

η απόφαση (stress on -φό-: a-PO-fa-si)

When you add an enclitic pronoun like μου (my), a rule of Greek accentuation often shifts or adds an accent to keep the stress from getting too far from the end. The result is:

η απόφασή μου (a-po-fa-SÍ mu)

So:

Without enclitic: η απόφαση
With enclitic: η απόφασή μου / σου / του / της

You will see the same pattern with many nouns and adjectives when followed by short enclitic pronouns (μου, σου, του, της, μας, σας, τους).

Could I say η απόφαση μου without the accent on the last syllable?

In correct writing, you should write:

η απόφασή μου (with the accent on the last -σή)

Writing η απόφαση μου without that accent is considered spelling incorrect, even if people would understand you.

In casual handwriting or online messages, natives sometimes omit accent corrections, but in proper Greek orthography the extra accent is required in combinations like απόφασή μου.

What is the difference between σίγουρος and βέβαιος? Could I say είμαι εκατό τοις εκατό βέβαιος?

You can definitely say:

Είμαι εκατό τοις εκατό βέβαιος για την απόφασή μου σήμερα.

Both σίγουρος and βέβαιος can mean “sure, certain”.

Nuance (very small):

σίγουρος is more common in everyday speech.
βέβαιος can sound a bit more formal or written, or “firmly certain”.

In many contexts they are interchangeable:

Είμαι σίγουρος / βέβαιος ότι έχεις δίκιο.

Is the word order fixed? Could I move σήμερα or εκατό τοις εκατό somewhere else?

Greek word order is relatively flexible. All of these are possible and natural, with slight changes in emphasis:

Είμαι εκατό τοις εκατό σίγουρος για την απόφασή μου σήμερα.
Neutral emphasis; today is when you are sure.
Σήμερα είμαι εκατό τοις εκατό σίγουρος για την απόφασή μου.
Emphasis on σήμερα (today, as opposed to some other day).
Είμαι σίγουρος εκατό τοις εκατό για την απόφασή μου σήμερα.
Slight emphasis on 100%; still natural.

Greek usually keeps the verb είμαι near the beginning, but adverbs like σήμερα can move quite freely.

Could I drop μου and just say για την απόφαση σήμερα?

Yes, but the meaning changes:

για την απόφασή μου σήμερα = about my decision today
για την απόφαση σήμερα = about the decision today (not specifically mine)

The possessive μου clearly marks that it is your own decision you are sure about. Without μου, you’re talking about some decision that both speaker and listener know about from context.

Where does the possessive pronoun μου usually go in noun phrases like this? Before or after the noun?

With a definite article, the usual place of μου is after the noun:

η απόφασή μου = my decision
το σπίτι μου = my house
ο φίλος μου = my (male) friend

You can put it before for stylistic or emphatic reasons, but that is less standard and often sounds poetic or very marked:

η δικιά μου απόφαση = my decision (as opposed to yours)

So the normal pattern here is η / την απόφασή μου, not μου η απόφαση.

Is εκατό τοις εκατό only used with σίγουρος, or can I use it in other contexts?

You can use εκατό τοις εκατό (or 100%) with many adjectives or nouns, just like in English:

Είμαι εκατό τοις εκατό σίγουρος. – I’m 100% sure.
Είμαι εκατό τοις εκατό ικανοποιημένος. – I’m 100% satisfied.
Είμαι εκατό τοις εκατό Έλληνας. – I’m 100% Greek.
Είναι εκατό τοις εκατό φυσικό προϊόν. – It’s 100% natural product.

So it’s a general intensifier meaning completely, fully, totally.

How do you pronounce Είμαι εκατό τοις εκατό σίγουρος για την απόφασή μου σήμερα?

Approximate phonetic transcription (standard Greek):

Είμαι → /ˈime/
εκατό → /ekaˈto/
τοις → /tis/
εκατό → /ekaˈto/
σίγουρος → /ˈsiɣuros/
για → /ʝa/ (like “ya”)
την → /tin/
απόφασή → /aˌpofaˈsi/ (stress on last syllable here)
μου → /mu/
σήμερα → /ˈsimera/

Said smoothly:

/ˈime ekaˈto tis ekaˈto ˈsiɣuros ʝa tin apofaˈsi mu ˈsimera/

Is σήμερα strictly necessary, or can I leave it out?

You can definitely leave it out:

Είμαι εκατό τοις εκατό σίγουρος για την απόφασή μου.

This just removes the time reference.

With σήμερα → I am 100% sure today (maybe I wasn’t before, or I might change later).
Without σήμερα → a general statement of current certainty, without highlighting “today” as special.