Otobüs geç kalırsa, ben yürürüm.

Geç kalmak is a very common Turkish expression meaning to be late.

• geç = late
• kalmak = to remain / to stay

But together, geç kalmak functions like one unit: to be late.

So in this sentence, Otobüs geç kalırsa means if the bus is late.

Kalırsa can be broken down like this:

• kal- = verb stem from kalmak
• -ır = aorist marker
• -sa = conditional marker, meaning if

So:

• kalır = it is / it becomes / it tends to be / it will be, depending on context
• kalırsa = if it is / if it becomes

Inside geç kalmak, this gives the meaning if it is late.

In this sentence, Turkish uses the aorist + conditional pattern:

• kalırsa
• yürürüm

This combination is very common for general conditions, habits, or likely results:

• If X happens, Y happens / I do Y

So kalırsa sounds like if it is late / if it ends up being late, not just a one-off hypothetical form.
Kalsa is also grammatical in other contexts, but it gives a slightly different feel and is not the standard choice for this kind of general conditional sentence.

Yürürüm is in the aorist.

It is built from:

• yürü- = walk
• -r = aorist marker
• -üm = I

So yürürüm means:

• I walk
• I will walk
• I usually walk

The exact meaning depends on context. In this sentence, it is most naturally understood as:

• I’ll walk or
• I walk (in that situation)

Turkish often uses the aorist in conditional sentences like this to express:

• general truths
• usual reactions
• expected results

So Otobüs geç kalırsa, ben yürürüm has the sense of:

• If the bus is late, I walk
• If the bus is late, I’ll walk
• If the bus is late, I usually walk

The aorist makes it sound natural and general, not overly specific.

Yes, yürürüm already means I walk / I’ll walk, so ben is not required.

Turkish often drops subject pronouns when the verb ending already shows the subject clearly.

So you could also say:

• Otobüs geç kalırsa, yürürüm.

Adding ben gives extra emphasis or contrast, like:

• If the bus is late, I walk.
• As for me, I’ll walk.

Yes. A very natural version is:

• Otobüs geç kalırsa, yürürüm.

This is probably the more neutral everyday form.

The version with ben is still correct; it just feels a bit more emphatic.

Here, otobüs is the subject of the first clause:

• The bus is late

Subjects in Turkish are normally in the basic dictionary form, with no extra case ending.

So:

• otobüs = the bus / a bus, as subject

You would use other endings only if the noun had a different role:

• otobüsü = the bus, as a direct object
• otobüse = to the bus

But here it is simply the thing doing the action of being late, so otobüs is correct.

Turkish does not have articles like the and a/an.

So otobüs can mean:

• the bus
• a bus

The exact meaning comes from context. In this sentence, English would usually say the bus, but Turkish just says otobüs.

Yes, it can be reversed.

Turkish often puts the if-clause first:

• Otobüs geç kalırsa, ben yürürüm.

But you can also say:

• Ben yürürüm, otobüs geç kalırsa.

Putting the conditional clause first is very common and feels natural because it sets up the situation before giving the result.

The comma separates the two clauses:

• Otobüs geç kalırsa = the condition
• ben yürürüm = the result

It works like English:

• If the bus is late, I walk / I’ll walk.

In writing, the comma is common and helpful here. In speech, there is usually a small pause.

Yes, by itself kalmak often means to stay, to remain, or to be left.

But in Turkish, many meanings come from fixed combinations.
With geç, it becomes:

• geç kalmak = to be late

So you should learn geç kalmak as a full expression, not by translating each word separately every time.

Yes. Because of the aorist, it can describe either:

• a usual pattern: If the bus is late, I walk
• a likely future decision: If the bus is late, I’ll walk

Both are possible. Context tells you which meaning is intended.

You can negate either clause depending on what you mean.

For example:

• Otobüs geç kalmazsa, ben yürümem. = If the bus isn’t late, I don’t walk / I won’t walk.

Or:

• Otobüs geç kalırsa, ben yürümem. = If the bus is late, I don’t walk / I won’t walk.

The negative forms are:

• kalmazsa = if it is not late
• yürümem = I do not walk / I will not walk

A rough pronunciation is:

• yü-rü-rüm

A few helpful notes:

• ü is a front rounded vowel, somewhat like the vowel in German ü or French u
• each vowel is pronounced clearly
• Turkish spelling is very regular, so it is pronounced much as it is written

The stress in Turkish can vary, but for learners the important thing is to pronounce each syllable clearly and keep the ü sound distinct from u.