Learn Hardcore Czech: Moje maminka dnes není tak unavená.

můj – used with masculine singular nouns (e.g. můj bratr – my brother)
moje – used with feminine and neuter singular nouns (e.g. moje maminka, moje auto) and also as a general form in speech
má – also feminine singular, like moje, but more formal/literary; in everyday speech moje is more common (e.g. má matka sounds a bit more bookish than moje maminka)

Here maminka is feminine, so moje is the natural everyday choice: moje maminka.

Why is it maminka and not matka? Is there a difference?

Yes:

maminka – affectionate, familiar; like mom / mummy / mum
máma – also friendly, like mum / mom, a bit more neutral than maminka
matka – more formal, neutral, or cold; like mother, and can even sound harsh in some contexts

In everyday speech about your own mother, maminka or máma is much more common than matka.
So Moje maminka… corresponds naturally to My mom….

Why is dnes (today) in the middle of the sentence? Can it go somewhere else?

Czech word order is flexible. Dnes can move without changing the basic meaning, only the emphasis:

Moje maminka dnes není tak unavená.
– Neutral: simply stating how she is today.
Dnes moje maminka není tak unavená.
– Slight emphasis on today (today, unlike other days).
Moje maminka není dnes tak unavená.
– Also possible; can make dnes feel slightly more stressed.

All three are grammatically correct. The original order is a very natural, neutral choice.

Why is there no word like “the” or “a” before maminka?

Czech does not use articles. There is no direct equivalent of English a / an / the.

So:

Moje maminka can mean my mom, my mother, the my mom (if that existed) – you simply rely on context.
You never add any extra word for the or a in front of maminka.

The possessive moje already makes it specific enough.

What does není mean exactly, and how is it formed?

není is the 3rd person singular negative form of být (to be) in the present tense.

Present tense of být:

jsem – I am
jsi – you are (sg.)
je – he / she / it is
jsme – we are
jste – you are (pl./formal)
jsou – they are

Negative forms:

nejsem, nejsi, není, nejsme, nejste, nejsou

So je → není (he/she/it is → is not).
In the sentence, není refers to moje maminka: my mom is not.

What does tak mean here? Is it “so” or “as”?

In this sentence tak works as a degree word:

tak unavená ≈ so tired / that tired / as tired

On its own, Moje maminka dnes není tak unavená most naturally feels like:

My mom is not so tired today
or
My mom is not as tired today (as usual / as before) – the comparison is usually understood from context, even if jako včera (as yesterday) is not said.

You could make the comparison explicit:

Moje maminka dnes není tak unavená jako včera.
– My mom is not as tired today as she was yesterday.

Why does unavená end with -á and not -ý like unavený?

Adjectives in Czech agree with the noun in gender, number, and case.

The base adjective is:

unavený – tired (masculine)
unavená – tired (feminine)
unavené – tired (neuter or plural in some contexts)

Since maminka is feminine singular, the adjective must be feminine singular nominative:
maminka je unavená → moje maminka dnes není tak unavená.

If you talked about a father (tatínek, masculine), you would say:

Můj tatínek dnes není tak unavený.

Could I leave out moje and just say Maminka dnes není tak unavená?

Yes, that is very natural in Czech.

With close family members, possession is often obvious from context, so the possessive moje is frequently omitted. For example:

Maminka dnes není tak unavená.
– In context, usually understood as My mom isn’t so tired today.

If you specifically need to make clear that you mean your mother (and not, say, someone else’s), you can keep moje.

Is the position of tak fixed, or can I move it?

With this kind of adjective phrase, tak normally stands directly before the adjective:

tak unavená – so / that / as tired

You would not normally separate them. You would not say:

✗ není unavená tak dnes

Instead, keep them together:

není tak unavená dnes
dnes není tak unavená

So tak is tightly connected to the adjective unavená.

How would I say “My mom is very tired today” instead of “not so tired”?

You change the negation and the degree word:

Moje maminka je dnes velmi unavená. – My mom is very tired today.
(velmi = very, quite formal, but common and correct)

More colloquial options:

Moje maminka je dnes hodně unavená. – very tired / really tired
Moje maminka je dnes strašně unavená. – terribly tired (informal, emphatic)

The structure is the same; you just remove the ne- and replace tak with another intensifier.

How is the sentence pronounced, especially dnes and není?

Approximate pronunciation:

Moje – [MO-ye], stress on Mo
maminka – [MA-min-ka], stress on Ma
dnes – [dnes] (the d and n blend a bit; no vowel between them)
není – [NE-nyi], the í is long, stress on Ne
tak – [tak]
unavená – [OO-na-ve-naa], stress on u (first syllable), long á at the end

In Czech, the stress is almost always on the first syllable of each word.