Výměna začíná dnes.

Výměna is a noun. In this sentence, it is in the nominative singular, because it is the subject of the sentence — the thing that starts.

It is also a feminine noun, which you can often guess from the ending -a.

Začíná is the 3rd person singular present tense form of začínat (to begin / to start).

It is in 3rd person singular because the subject, výměna, is singular:

• výměna začíná = the exchange starts
• if the subject were plural, the verb would change:
  • výměny začínají = the exchanges start

Czech often does not use subject pronouns when they are not needed.

In English, you might say:

• It starts today

But in Czech, the subject is already named:

• Výměna začíná dnes.

So there is no need to add a separate word for it.

Czech does not have articles like a, an, and the.

So výměna can mean:

• an exchange
• the exchange

The exact meaning depends on the context.

Dnes is an adverb, meaning today.

Because it is an adverb, it does not change for case, gender, or number. It simply gives time information about the action:

• začíná dnes = starts today

So no, dnes is not a noun in a case form here.

Yes, Czech word order is quite flexible.

Výměna začíná dnes. is a natural neutral order, but you can also say:

• Dnes začíná výměna.
• Výměna dnes začíná.

These all mean roughly the same thing, but the emphasis changes a little:

• Dnes začíná výměna. emphasizes today
• Výměna začíná dnes. is a straightforward neutral statement

So the end position is normal, but not the only possibility.

Grammatically, začíná is present tense.

But just like in English, Czech present tense can be used for something happening now or for a scheduled / expected event:

• Výměna začíná dnes. = The exchange starts today / is starting today

So although the form is present, the meaning can refer to something beginning today, including a planned event.

A helpful approximate pronunciation is:

VEE-mnye-na

A few important details:

• ý is a long ee sound
• mě sounds roughly like m-nye
• the stress in Czech is normally on the first syllable, so:
  • VÝ-mě-na

So the main stress is on vý.

These marks are important in Czech.

• á is a long a
• ý is a long y/i-type vowel
• ě is special: after some consonants, it changes the pronunciation of the consonant and often adds a y-like sound

In výměna:

• mě is pronounced roughly like mye / mnye, not like plain me

In začíná:

• á at the end is long, so the final syllable is stretched slightly

These marks are not optional in normal Czech spelling.

You probably can, but it is less neutral and may sound marked depending on the context.

The most natural versions are usually:

• Výměna začíná dnes.
• Dnes začíná výměna.

Putting dnes at the very end or near the beginning is more common than Začíná výměna dnes, unless you are aiming for a particular emphasis.

Začínat is imperfective.

Its perfective partner is usually začít.

This matters because:

• imperfective present can describe an action as ongoing, habitual, or simply as a present-time event
• perfective present in Czech often has a future meaning

Here, začíná from začínat works naturally for a statement like starts today / is starting today.

A related form would be:

• Výměna začne dnes.

That also means The exchange will start today, but it sounds a bit more clearly future-oriented or event-focused.

You would need to make both the noun and the verb plural:

• Výměny začínají dnes.

Changes:

• výměna → výměny (plural)
• začíná → začínají (3rd person plural)

So Czech, unlike English in some cases, clearly shows the singular/plural agreement on the verb.