Když nemám čas na oběd, dám si jen malou svačinu a sklenici džusu.

Here když means when.

It introduces a subordinate clause: Když nemám čas na oběd = When I don’t have time for lunch.

A useful thing to know is that když can also mean whenever depending on context, especially in sentences about repeated situations, as in this example.

Czech often leaves out subject pronouns when they are already clear from the verb ending.

• nemám = I don’t have
• dám si = I will have / I have for myself

Because the -m ending already shows I, adding já is usually unnecessary.

You could say Když já nemám čas..., but that would add emphasis, something like When I don’t have time... with stress on I.

Nemám is:

• ne- = negation, not
• mám = I have

So nemám means I don’t have.

The verb is mít = to have.

Its 1st person singular present form is:

• mám = I have
• nemám = I do not have

In Czech, čas na něco means time for something.

So:

• čas na oběd = time for lunch
• čas na kávu = time for coffee
• čas na odpočinek = time for rest

The preposition na is required in this expression.

Also, oběd is in the accusative after na here.

Čas is in the accusative because it is the direct object of nemám.

• mám čas = I have time
• nemám čas = I don’t have time

So in the clause Když nemám čas na oběd, the structure is basically:

• nemám = I don’t have
• čas = the thing I don’t have
• na oběd = for lunch

Dám si is a very common Czech expression used for food and drink. It literally comes from dát si, which is roughly to give oneself, but in natural English it usually means:

• I’ll have
• I have
• I’ll take

So:

• Dám si kávu. = I’ll have a coffee.
• Dám si jen malou svačinu. = I’ll just have a small snack.

It is one of the most useful everyday expressions in Czech restaurants and casual conversation.

Si is the short form of the reflexive pronoun used in the dative.

In dát si, it suggests doing something for oneself. That is why this construction is so common with eating and drinking.

So:

• dám si oběd = I’ll have lunch
• dám si čaj = I’ll have tea

Without si, dám usually means simply I give:

• Dám knihu Petrovi. = I’ll give the book to Petr.

So dám si and dám are not the same.

This is a very common learner question.

• dát si is perfective
• dávat si is imperfective

In this sentence, dám si is natural because it presents the action as the usual result or response in that situation: when that happens, I have a snack.

English often uses the simple present for this kind of repeated meaning, but Czech can use the perfective present very naturally in these when X happens, I do Y patterns.

Compare:

• Když nemám čas, dám si svačinu. = When I don’t have time, I have a snack.
• Dávám si svačinu každý den. = I have a snack every day.

The imperfective dávám si would emphasize the ongoing or habitual nature more directly, but dám si is completely natural here.

Because dát si takes a direct object in the accusative, and both the adjective and noun must match that case.

The noun is svačina = snack, which is feminine.

So:

• nominative: malá svačina
• accusative: malou svačinu

That is why the ending changes on both words:

• malá → malou
• svačina → svačinu

No. Svačina is not the same as oběd.

• oběd = lunch
• svačina = snack, often a light meal between main meals

In Czech culture, svačina is a very common word. It can mean a snack eaten in the morning or afternoon, often something smaller than a full meal.

So in this sentence, the idea is:

If I don’t have time for lunch, I just have a small snack instead.

This phrase has two different case relationships:

1. sklenici is accusative singular, because it is another direct object of dám si
2. džusu is genitive singular, because after a container word like glass, Czech often uses of + something logic

So:

• sklenice = glass
• sklenici = accusative form
• džus = juice
• džusu = of juice

Literally, the phrase is:

a glass of juice

This is very common in Czech:

• šálek čaje = a cup of tea
• láhev vody = a bottle of water
• sklenice mléka = a glass of milk

Because it is in the accusative singular here.

The basic dictionary form is sklenice. But after dám si, it functions as a direct object, so it changes form:

• nominative: sklenice
• accusative: sklenici

This is a normal declension pattern for many feminine nouns ending in -e or -ice.

Jen means only or just.

Here it limits the action:

• dám si jen malou svačinu a sklenici džusu
• I just have only a small snack and a glass of juice

In natural English, we usually say I just have a small snack and a glass of juice or I only have...

In Czech, jen is very flexible in position, but here it naturally modifies the whole idea of having only that and nothing more.

Yes, Czech word order is flexible, although not random.

For example, you could also hear:

• Když nemám čas na oběd, jen si dám malou svačinu a sklenici džusu.
• Když nemám čas na oběd, dám si malou svačinu a jen sklenici džusu.

But these versions shift the emphasis slightly:

• jen si dám emphasizes I just have
• jen sklenici džusu emphasizes only a glass of juice

The original sentence is neutral and natural.

Because Když nemám čas na oběd is a subordinate clause, and Czech normally separates subordinate clauses with a comma.

So the structure is:

• subordinate clause: Když nemám čas na oběd
• main clause: dám si jen malou svačinu a sklenici džusu

This comma is required in standard written Czech.

Not in the same meaning.

• čas na oběd = time for lunch
• něco k obědu = something for lunch / something to eat as lunch

So:

• nemám čas na oběd = I don’t have time for lunch
• dám si polévku k obědu = I’ll have soup for lunch

They are different expressions.

Džus is a normal, common Czech word for juice. It is a borrowing, but it is fully standard and widely used.

You may also see šťáva, which literally means juice too, but the nuance can differ depending on context.

In everyday speech, džus is very common for a drink like orange juice, apple juice, and so on.

So sklenici džusu is perfectly natural Czech.