Dayım geç kalırsa eniştem onu istasyondan alacak.

The ending -ım means my.

• dayı = maternal uncle
• dayım = my maternal uncle

This is a possessive ending, and Turkish uses it very often instead of a separate word like my.

Not exactly. Turkish often distinguishes family relationships more specifically than English.

• dayı = your mother’s brother
• amca = your father’s brother

So dayım is specifically my mother’s brother, not just any uncle.

Eniştem means my brother-in-law, but in Turkish this can refer to a more specific relationship than in English.

• enişte usually refers to the husband of your sister, aunt, or female relative
• eniştem = my brother-in-law / my aunt’s husband / my sister’s husband, depending on context

In this sentence, the exact family connection is understood from context, but grammatically it is enişte + -m = my enişte.

Geç kalırsa means if he is late or if he arrives late.

It breaks down like this:

• geç kalmak = to be late
• kalır = he/she stays / is / ends up, here as part of the expression
• -sa = if

So:

• kalırsa = if he/she is / if he/she ends up

Together, geç kalırsa is the normal Turkish way to say if he is late.

Because Turkish usually leaves out subject pronouns when they are understood.

In kalırsa, the subject can be understood from context. Here it refers to dayım.

So Turkish does not need to say something like o geç kalırsa unless the speaker wants extra emphasis or contrast.

That is very normal in Turkish.

In conditional clauses with if, Turkish often uses a non-future form even when the meaning is future in English.

So:

• Dayım geç kalırsa = If my uncle is late
• not literally If my uncle will be late

English also often does this: we say If he is late, not usually If he will be late.

Onu means him / her / it as a direct object.

• o = he / she / it
• onu = him / her / it

The ending -nu is the accusative marker here. It shows that this person is the object of the verb alacak.

So in this sentence, onu means him, referring to dayım.

The ending -dan / -den / -tan / -ten means from.

• istasyon = station
• istasyondan = from the station

So onu istasyondan alacak means he will pick him up from the station.

Because the meaning here is to pick someone up from a place.

With almak, Turkish can use different case endings depending on meaning:

• bir şeyi almak = to take/get something
• birini bir yerden almak = to pick someone up from somewhere

So:

• istasyondan almak = pick up from the station

Using istasyona would suggest movement to the station, which is a different idea.

Literally, alacak is he/she will take or will get, from almak.

But in context, Turkish birini bir yerden almak often means:

• to pick someone up
• to collect someone

So eniştem onu istasyondan alacak naturally means my brother-in-law will pick him up from the station.

Yes, Turkish word order is flexible, but this sentence uses a very natural neutral order.

• Dayım geç kalırsa = if my uncle is late
• eniştem onu istasyondan alacak = my brother-in-law will pick him up from the station

A more literal structure is:

[condition] [subject] [object] [place/source] [verb]

You could move parts around for emphasis, but the verb usually stays at the end:

• Dayım geç kalırsa eniştem istasyondan onu alacak.
• Eniştem, dayım geç kalırsa, onu istasyondan alacak.

These are possible, but the original sounds very natural.

We know from context and logic.

The sentence says:

• If my uncle is late, my brother-in-law will pick him up from the station.

The person likely being picked up is the same person who might be late: dayım.

Grammatically, onu could mean him/her/it, but in this sentence the most natural interpretation is him = dayım.

Yes, very much so.

Geç kalmak is the standard Turkish expression for to be late.

Examples:

• Okula geç kaldım. = I was late to school.
• Toplantıya geç kalma. = Don’t be late to the meeting.

Even though geç means late and kalmak often means to remain/stay, together they function as a set expression meaning to be late.

Because Turkish uses vowel harmony and sometimes consonant changes.

For example:

• -ım / -im / -um / -üm all mean the same possessive idea, but the vowel changes to match the word
• -dan / -den / -tan / -ten all mean from, but the form changes depending on the sounds in the word

So:

• dayı + -ım → dayım
• enişte + -m → eniştem
• istasyon + -dan → istasyondan

These changes are normal and are one of the main patterns in Turkish grammar.