Na piknik do parku vezmeme deku, vodu a ubrousky, abychom si mohli dát sendviče.

They express two different ideas:

• na piknik = for a picnic / on a picnic
• do parku = to the park

So together, it means something like to the park for a picnic.

The cases are important too:

• na + accusative: here piknik is accusative singular, but it looks the same as nominative because piknik is masculine inanimate.
• do + genitive: park becomes parku.

Czech often uses na with events or activities, and do with movement into a place.

Vezmeme is from the verb vzít (to take), which is perfective.

In Czech, perfective verbs use present-tense forms to express a future result. So:

• vezmeme literally looks like a present form
• but it means we will take

That is normal in Czech. Compare:

• bereme = we are taking / we take (imperfective)
• vezmeme = we will take (perfective)

So vezmeme is not present in meaning here; it is future.

Because they are direct objects of vezmeme, so they must be in the accusative case.

Both deka and voda are feminine singular nouns ending in -a, and their accusative singular changes to -u:

• deka → deku
• voda → vodu

This is a very common pattern in Czech.

It is also a direct object, so it is in the accusative. But ubrousky is the plural of ubrousek, a masculine inanimate noun.

For many masculine inanimate nouns, the nominative plural and accusative plural are the same:

• nominative plural: ubrousky
• accusative plural: ubrousky

So it really is declined correctly; it just happens to have the same form in both cases.

Because abychom si mohli dát sendviče is a subordinate clause.

In Czech, subordinate clauses are normally separated by commas, and this is much more regular than in English. Since aby introduces a purpose clause (so that / in order that), a comma is required before it.

Abychom means so that we or in order for us to.

It is built from:

• aby = so that
• bychom = a 1st person plural conditional element (we would)

Together, abychom introduces a purpose clause with we as the subject.

So:

• abychom si mohli dát sendviče = so that we could / can have sandwiches

In standard Czech, abychom is the correct written form.

No, it is not past tense in meaning here.

In abychom si mohli dát sendviče, the combination abychom + mohli is used to express purpose: so that we can / will be able to have sandwiches.

The form mohli is the l-participle of moci (to be able to / can), and with abychom it forms the kind of clause English often translates with can, could, or be able to, depending on style.

So although mohli may look past-like, here the whole clause refers to the intended future situation.

Yes, dát usually means to give, but dát si is a very common idiomatic expression.

With food and drink, dát si often means:

• to have
• to eat
• to drink

So:

• dát si sendviče = to have/eat sandwiches

The word si is the dative reflexive pronoun, roughly for oneself, but in many expressions you should learn dát si as a set phrase.

Examples:

• Dám si kávu. = I’ll have a coffee.
• Dáme si oběd. = We’ll have lunch.

In this expression, si is normally part of the meaning.

• dát sendviče would usually mean to give sandwiches
• dát si sendviče means to have/eat sandwiches

So omitting si would change the meaning a lot. This is why learners often need to memorize dát si as a unit.

Because sendviče is the accusative plural of sendvič.

Here it is the object of dát si, and the sentence is talking about more than one sandwich.

Since sendvič is masculine inanimate, its accusative plural is the same as its nominative plural:

• singular: sendvič
• plural: sendviče

So dát si sendviče means to have/eat sandwiches.

Yes. Czech word order is flexible, and several versions are possible.

The original sentence starts with Na piknik do parku to set the scene first. That sounds natural and smooth. But you could also say:

• Vezmeme na piknik do parku deku, vodu a ubrousky...
• Deku, vodu a ubrousky vezmeme na piknik do parku...

The basic meaning stays the same, but the emphasis shifts slightly depending on what comes first. Czech uses word order much more for emphasis and information structure than English does.