Learn Hardcore Czech: Když jdu pomalu do školy, nemám čas, ale když jedu rychle autobusem, mám čas. - When I walk slowly to school, I don’t have time, but when I go by bus quickly, I have time.

jít – jdu = to go on foot, to walk (in one direction, at this moment)
jet – jedu = to go by vehicle (bus, car, train, bike, etc., also one direction, now)

So:

Když jdu pomalu do školy… = When I walk slowly to school…
…ale když jedu rychle autobusem… = …but when I go by bus quickly…

English often uses just go, but Czech needs you to choose the correct verb depending on whether you’re walking (jdu) or travelling by some transport (jedu).

Why is it do školy and not do škola or do školu?

The preposition do (“to, into”) always takes the genitive case in Czech.

The noun is škola (school) – nominative singular.
Genitive singular of škola is školy.

So with do you must say:

do školy = to school (literally “into-of school”)

You cannot say do škola or do školu; those are wrong case forms after do.

What exactly does autobusem mean, and why is it in that form?

Autobusem is instrumental case of autobus (bus).

The instrumental is often used for means of transport and can be translated as “by …”:

autobus → autobusem = by bus
vlak → vlakem = by train
auto → autem = by car

So:

jedu autobusem = I go by bus
jedu rychle autobusem = I go quickly by bus

The instrumental here answers “by what means?” – by bus.

Why is the word order jdu pomalu do školy and jedu rychle autobusem? Could the adverbs go in other positions?

Yes, the word order of pomalu (slowly) and rychle (quickly) is quite flexible. In Czech, adverbs of manner usually stand after the verb, but can be moved for emphasis or rhythm.

All of these are grammatically possible:

Když jdu pomalu do školy…
Když pomalu jdu do školy… (slight emphasis on slowly)
Když jdu do školy pomalu… (emphasis at the end)

Similarly:

když jedu rychle autobusem…
když jedu autobusem rychle…
když rychle jedu autobusem… (stronger focus on quickly)

The original word order is natural and neutral. Moving the adverb often changes the emphasis more than the basic meaning.

Why is když used here, and not kdy or až, when all can relate to “when”?

These three words are very different:

když = when / whenever / if (introduces a subordinate clause stating a condition or time)
- Když jdu pomalu do školy, nemám čas.
  When I walk slowly to school, I don’t have time.
kdy = when? (a question word)
- Kdy jdeš do školy? = When are you going to school?
až = when / once (at the moment something future or later happens)
- Až půjdu do školy, zavolám ti. = When I go to school / Once I’m going to school, I’ll call you.

In your sentence, we have a general condition/time situation: When I do X, Y happens. That’s exactly what když is for.
Using kdy or až here would be incorrect.

Why are there commas before nemám čas and ale in this sentence?

The commas follow standard Czech rules for separating clauses and conjunctions:

Když jdu pomalu do školy, nemám čas, ale když jedu rychle autobusem, mám čas.

Breakdown:

Když jdu pomalu do školy,
– This is a subordinate clause introduced by když. In Czech, a comma usually closes such a clause.
nemám čas, ale
– ale (“but”) is a coordinating conjunction joining two main clauses:
- nemám čas
- (já) mám čas
  – In Czech you normally put a comma before ale.
…ale když jedu rychle autobusem, mám čas.
– The second když clause (když jedu rychle autobusem) is again separated from the main clause (mám čas) by a comma.

So the commas mark the boundaries between the main clause and the když-clauses, and also precede ale.

Why isn’t there a pronoun já (“I”) anywhere? How do we know the subject is “I”?

Czech verbs include information about person and number in their endings. That means you usually don’t need to say já (I), ty (you), etc.

jdu = 1st person singular (I go, I am going)
jedu = 1st person singular (I go by vehicle)
mám / nemám = I have / I don’t have

So já is understood from the verb form. You only use já when you want to emphasize it:

Já jdu pomalu do školy. = I (as opposed to someone else) walk slowly to school.
Já nemám čas. = I don’t have time (but maybe someone else does).

Is this sentence talking about now, or about a general/habitual situation? Why is the present tense used?

Czech present tense can express:

Action happening now
Regular / habitual actions
General truths

Your sentence is best understood as habitual or general:

Když jdu pomalu do školy, nemám čas…
…ale když jedu rychle autobusem, mám čas.

It describes what usually happens in these situations.

In English you also use the present simple for this kind of general rule:

When I go slowly to school, I don’t have time, but when I go quickly by bus, I have time.
(understood as “whenever that happens”)

So the Czech present tense here is completely natural for a general pattern.

Is nemám čas the normal way to say “I don’t have time”? Are there other common variants?

Yes, nemám čas is the basic, very common way to say “I don’t have time.”

You can vary it a bit depending on what you want to express:

Nemám čas. = I don’t have time. (general)
Nemám čas jít do školy. = I don’t have time to go to school.
Nemám na to čas. = I don’t have time for that / for it.
Já na to nemám čas. = I don’t have time for that. (emphasis on já)
Nestíhám. (colloquial) = I can’t keep up / I’m not managing (time-wise).

You might occasionally see nemám času (genitive), but in modern everyday speech nemám čas is much more common and neutral.

Could the word order be changed to something like Já čas nemám or Mám čas, když jedu rychle autobusem?

Yes. Czech word order is fairly flexible and often used to change the emphasis rather than the core meaning.

All of these are possible:

Nemám čas. (neutral)
Já nemám čas. (emphasis on I)
Já čas nemám. (strong emphasis on I and on not having it)
Čas nemám. (emphasis on time as the thing you lack)

You can also flip the order of main and když-clauses:

Když jedu rychle autobusem, mám čas. (original)
Mám čas, když jedu rychle autobusem.
→ similar meaning, slightly more emphasis on mám čas as the main point.

So yes, variations like Mám čas, když jedu rychle autobusem are perfectly correct; they just shift the information focus.

Why is it jdu do školy and not chodím do školy if this sounds like a repeated situation?

Czech has two different verbs for “to go (on foot)”:

jít – jdu: one-way, this specific trip, concrete motion
chodit – chodím: repeated or habitual going, or “to attend”

Compare:

Jdu do školy. = I’m on my way to school (now, this time).
Chodím do školy. = I go to school (regularly / I attend that school).

In your sentence:

Když jdu pomalu do školy, nemám čas…

This can be understood as:

whenever I’m walking (on a given occasion) on my way to school slowly, I don’t have time.

If you said:

Když chodím pomalu do školy, nemám čas…

it would sound more like a general habitual style of walking to school (every time you go to school, you tend to walk slowly). Both can be possible depending on the context, but jdu keeps the focus more on each particular journey.

Could I say Když jedu autobusem rychle instead of když jedu rychle autobusem? Does it change the meaning?

You can say both:

když jedu rychle autobusem
když jedu autobusem rychle

The meaning (I go quickly by bus) stays almost the same, but:

když jedu rychle autobusem
– neutral, with rychle closely tied to jedu.
když jedu autobusem rychle
– a bit more emphasis on rychle at the end; stylistically slightly less common, but fine.

Czech often places key or emphasized information towards the end of the clause. So the last position is usually the most stressed.