Zářez se v kůrce otevře.

The subject is zářez.

It is in the nominative singular, and the verb otevře is also 3rd person singular, so they agree:

• zářez = the cut / incision / notch
• otevře = opens / will open

So grammatically, the cut is what is doing the action.

Czech normally has no articles.

So zářez can mean:

• a cut
• the cut

The exact choice in English depends on context. Czech leaves that unstated unless something else makes it clear.

Here se is a reflexive particle, but in this sentence it does not literally mean itself.

It is part of the verb otevřít se, which means to open or to open up in an intransitive sense.

Compare:

• otevřít něco = to open something
• otevřít se = to open

So:

• Dveře otevřely. would sound odd or incomplete in standard Czech.
• Dveře se otevřely. = The door opened.

Likewise here, zářez se otevře = the cut will open up.

Because se usually goes in the second position in a Czech clause.

So even though the dictionary form is otevřít se, in a real sentence you often get:

• Zářez se otevře.
• V kůrce se zářez otevře.

Not normally:

• Zářez otevře se.

This is a very common Czech word-order rule for short unstressed words like se, si, mi, ti, jsem, and so on.

Because otevřít se is a perfective verb.

In Czech, the present forms of a perfective verb usually refer to the future. So:

• otevře se = it will open
• not usually it is opening

If you want an ongoing or repeated present meaning, you would use the imperfective verb:

• otevírá se = it is opening / it opens

So the contrast is:

• zářez se otevře = the cut will open
• zářez se otevírá = the cut is opening

kůrce is in the locative singular.

That is because the preposition v means in here, and when v expresses location, it takes the locative:

• v domě = in the house
• v lese = in the forest
• v kůrce = in the bark / rind / crust

So the phrase tells you where the opening happens.

Because Czech is viewing the cut as opening within the bark layer, not simply on its surface.

So v fits the idea of something happening in / inside / within that material.

English sometimes uses in or on depending on perspective, but Czech here chooses v, which is very natural if the bark is understood as the material containing the cut.

Because the form kůrce comes from the noun kůrka.

• kůrka → locative singular v kůrce
• kůra → locative singular v kůře

So this sentence is built from kůrka, not kůra.

Depending on context, kůrka can mean crust, rind, or a smaller/thinner bark layer, while kůra is the more basic word for bark. In some technical or specialized contexts, either may appear depending on exactly what layer is meant.

Yes, Czech word order is fairly flexible, but the emphasis changes.

For example:

• Zářez se v kůrce otevře. = neutral
• V kůrce se zářez otevře. = emphasizes the location
• Zářez se otevře v kůrce. = also possible, with a slightly different flow

What usually stays the same is that se still wants to be near the second position in the clause.

Zářez is a noun meaning something like:

• cut
• slit
• incision
• notch

The best English word depends on the context. In a practical or technical context involving bark, cut or incision is often a good choice.

So it is not necessarily a huge cut; it can be a deliberate slit or notch made in a surface.

It is best understood as a middle/reflexive construction.

Czech often uses verb + se where English uses an intransitive verb:

• Dveře se otevřely. = The door opened.
• Okno se zavřelo. = The window closed.

So zářez se otevře is not a typical passive like is opened. It means the cut opens up, without focusing on an outside agent.

A few helpful points:

• á is a long vowel: zá-
• ů is also a long vowel: ků-
• c in Czech is pronounced ts
• ř is the famous Czech sound, something like an r mixed with a zh sound

Roughly:

• zářez ≈ zaa-rzhezz
• kůrce ≈ koor-tse

That is only an approximation for English speakers, but it gives the general idea. The hardest sound here is usually ř in zářez.