Learn Hardcore Czech: Mně je třicet let a sestra je o dva roky mladší. - I am thirty years old and my sister is two years younger.

✗ Já jsem třicet let – sounds like “I am thirty years (time period)” and is wrong for age.
✗ Mám třicet let – directly modeled on French/Spanish, sometimes heard from learners, but not idiomatic in Czech (it sounds foreign or childish).

Correct ways:

Je mi třicet (let). – most common.
Mně je třicet (let). – same meaning, different pronoun form and emphasis (see below).

What is the difference between mně, mi, and mě? Why is it Mně je třicet let here?

All three are forms of the pronoun já (I), but in different cases/uses.

já – nominative (subject): Já jsem učitel. – I am a teacher.
mě / mne – accusative or genitive (object):
- Vidíš mě? – Do you see me?
- Bez mě to nezvládneš. – Without me you won’t manage.
mi / mně – dative (“to me”, “for me”):
- Dej mi to. – Give it to me.
- Je mi / mně třicet let. – I am thirty.

Key points:

For age, you must use the dative: Je mi / mně třicet let.
mi is the short, very common spoken form.
mně is the full form, often used for emphasis or in careful/official writing.
mě is never correct in the age expression; it’s the wrong case.

In your sentence, Mně je třicet let slightly emphasizes “as for me” / “I’m the one who is thirty”.

Can I also say Je mi třicet let instead of Mně je třicet let? Which one is more natural?

Yes, you can absolutely say:

Je mi třicet let.

This is actually the most typical, neutral way to say your age.

Differences in feel:

Je mi třicet let. – very common, neutral, what you’ll hear most.
Mně je třicet let. – same grammar, but with a bit of emphasis on me, or a bit more formal/written style in some contexts.

Example:

Mně je třicet let a sestře je dvacet osm.
“I am thirty and my sister is twenty‑eight.” (slight contrast)

Why is it třicet let and not třicet roků or třicet roky?

All of these relate to the word “year”, but with different forms and usage.

rok – year (basic form)
roky – nominative/accusative plural of rok (used with 2, 3, 4; and in some other roles)
roků / let – genitive plural forms
- let is the usual word for “years” when talking about age or time span
- roků is also possible but sounds more literal / less idiomatic here

For age:

Je mi třicet let. – totally standard and most natural
Je mi třicet roků. – grammatically OK, but sounds heavier, more emphatic or old‑fashioned
✗ třicet roky – incorrect form here

So in everyday speech, use třicet let for “thirty years (old)”.

Why doesn’t the sentence say Moje sestra? Is sestra alone enough?

In Czech, you often omit the possessive pronoun (my, your, his, her, etc.) when context makes it obvious whose relative or thing it is.

So:

Mně je třicet let a sestra je o dva roky mladší.

Naturally reads as:

“I am thirty and (my) sister is two years younger.”

Using moje sestra:

Mně je třicet let a moje sestra je o dva roky mladší.

This is also correct, but:

It adds a bit of emphasis or contrast (e.g. not someone else’s sister).
In a neutral context where you are clearly talking about your own family, just sestra is completely normal.

What does the construction o dva roky mladší literally mean, and why is o used?

The phrase o dva roky mladší literally means:

“younger by two years”

Structure:

mladší – “younger”
o + [accusative] – “by (difference of) …”
dva roky – “two years” (accusative plural)

In Czech, when you express the difference in a comparison, you usually use:

o + accusative + comparative adjective

Examples:

Je o hlavu vyšší. – He is a head taller.
Je o pět kilo těžší. – She is five kilos heavier.
Je o dva roky mladší. – She is two years younger.

So o here is not “about” (like “talk about”), but “by” in the sense of “by a difference of”.

Can I change the word order and say Sestra je mladší o dva roky?

Yes, both word orders are correct:

Sestra je o dva roky mladší.
Sestra je mladší o dva roky.

Differences:

o dva roky mladší (pre‑positioned) is a very common pattern and sounds a bit more fixed, like a set phrase.
mladší o dva roky is also natural; it may lightly stress the quality “mladší” first and then specify “by two years”.

Both are perfectly acceptable. In everyday speech you’ll hear both; the version in your sentence is slightly more typical.

Why is it dva roky and not dvě roky?

The number 2 has different forms depending on the gender of the noun:

dva – with masculine and neuter nouns:
- dva muži – two men (masc.)
- dva stoly – two tables (masc.)
- dva roky – two years (masc.)
dvě – with feminine nouns (and some patterns of neuter in other contexts):
- dvě ženy – two women
- dvě hodiny – two hours

Rok (year) is masculine, so you must say:

dva roky, not ✗ dvě roky.

What form is mladší? Does it change for gender or number?

Mladší is the comparative of the adjective mladý (young → younger).

Important points:

As a comparative, mladší has the same form for masculine and feminine singular in this sentence:
- bratr je mladší – the brother is younger
- sestra je mladší – the sister is younger
It does change for:
- plural: mladší (same shape, but different agreement in context)
- case (when declined before a noun, etc.)

But in Sestra je o dva roky mladší, it stands alone, agrees with sestra, and the form mladší is exactly right for feminine singular here.

Could I say something like Je mi třicet a sestra má o dva roky míň instead?

Yes, this is a more colloquial / conversational way to say it:

Je mi třicet a sestra má o dva roky míň.
- Je mi třicet – I’m thirty.
- sestra má o dva roky míň – my sister has two years less (is two years younger).

Notes:

má o dva roky míň is idiomatic spoken Czech.
Grammatically, míň is a colloquial form of méně (less).
Your original sentence is a bit more neutral/standard:
- … a sestra je o dva roky mladší.

Both are fine; the original is slightly more “textbook”, the alternative more informal.

What cases are used in třicet let and dva roky?

třicet let
- třicet – numeral “thirty”
- let – genitive plural of rok (year) in the special time/age expression
- As a whole, “age” is usually expressed with [number] + let (genitive plural).
dva roky
- dva – numeral “two”
- roky – plural form of rok that here functions as accusative, governed by the preposition o (“by two years”).

So:

Age: Je mi třicet let. – number + let (genitive) is the standard pattern.
Difference with o: o dva roky – o + accusative to mark the amount of difference.

How is Mně je třicet let a sestra je o dva roky mladší pronounced, and is there any pronunciation trap for English speakers?

Approximate pronunciation (with stress always on the first syllable of each word):

Mně – like mnyeh (the mn is pronounced together; mě/mně sound the same)
je – ye
třicet – roughly TRZHI-tset
- ř is a special Czech sound (a rolled/“buzzing” r + zh), very tricky for learners
let – let
a – ah
sestra – SE-stra
je – ye
o – like British or without the r, just a pure o
dva – dva
roky – RO-ki
mladší – MLAD-shee

Main trap:

The sound ř in třicet.
Pronunciation of mě/mně: both are pronounced as [mňe], there is no difference in sound, only in spelling and grammar.