Na dlouhou cestu si moje sestra vždycky připraví dvě sušenky a jeden sendvič.

It is accusative singular.

• na can take either accusative or locative, depending on meaning.
• Here na dlouhou cestu means something like for a long journey / for a long trip, so Czech uses na + accusative.
• cesta is a feminine noun, so:
  • nominative: dlouhá cesta
  • accusative: dlouhou cestu

The adjective dlouhou matches cestu in gender, number, and case.

Because si is a clitic: a short unstressed word that usually goes in second position in the sentence.

In this sentence, the first whole chunk is Na dlouhou cestu, so si comes immediately after that:

• Na dlouhou cestu si moje sestra vždycky připraví...

It is not attached specifically to cestu. It is following the Czech second-position rule.

If you changed the word order, si would still try to stay in second position:

• Moje sestra si vždycky připraví...
• Vždycky si moje sestra připraví...

Here si is the dative reflexive pronoun, and it roughly means for herself.

So připravit si něco means:

• to prepare something for oneself
• sometimes also to get something ready for oneself

In English, this is often not translated directly, because English usually just says prepare.

So the idea is that the sister prepares the food for her own use, not for someone else.

This is about aspect.

• připravit = perfective
• připravovat = imperfective

připraví is the present form of a perfective verb. Perfective present forms often refer to the future, but with words like vždycky, they can describe repeated complete actions:

• every time there is a long trip, she gets everything prepared

So vždycky připraví means something like she always makes sure to prepare.

If you said vždycky připravuje, that would be more focused on the activity or habit in a less bounded way. Both can make sense, but připraví nicely presents each act as a completed whole.

Yes, má sestra is also possible.

Both mean my sister:

• moje sestra
• má sestra

The difference is mainly stylistic:

• má is the shorter form
• moje is the fuller form and can sound a little more emphatic, clearer, or more natural in some contexts

In everyday Czech, both are common.

Because svůj / svoje refers back to the subject of the sentence.

Here the subject is moje sestra. So svůj would refer to the sister’s own something, not my something.

• moje sestra = my sister
• svoje sestra would try to mean something like her own sister, which does not fit here

Compare:

• Moje sestra si připraví svůj sendvič.
  = My sister prepares her own sandwich.

But when you want to say my sister, you need moje / má, not svoje.

Because the numerals must match the gender of the noun.

• sušenka is feminine, so with 2 you use dvě
• sendvič is masculine inanimate, so with 1 you use jeden

So:

• dvě sušenky
• jeden sendvič

A useful pattern:

• dva = masculine
• dvě = feminine and neuter

Because after 2, 3, 4, Czech normally uses the noun in the plural nominative/accusative-type form, not the genitive plural.

So you get:

• dvě sušenky
• tři sušenky
• čtyři sušenky

But:

• pět sušenek
• šest sušenek

So sušenek would be used after 5 and above, not after dvě.

Yes, vždycky and vždy both mean always.

In this sentence, vždycky is very natural and common in everyday Czech.

A rough difference:

• vždycky = very common in speech
• vždy = often a bit shorter, sometimes more formal, literary, or used in fixed expressions

So here vždycky is exactly what many native speakers would say.

Yes. Czech word order is quite flexible.

This sentence begins with Na dlouhou cestu, which gives that phrase a bit of emphasis or makes it the starting point of the sentence.

Other possible orders include:

• Moje sestra si vždycky na dlouhou cestu připraví dvě sušenky a jeden sendvič.
• Moje sestra si na dlouhou cestu vždycky připraví dvě sušenky a jeden sendvič.

The meaning stays very similar, but the information focus changes slightly.

The main thing that does not change much is the position of si, because clitics still prefer second position.