Dün parkta eski bir arkadaşımla karşılaştım.

Because the most neutral Turkish sentence order usually puts the verb last.

In this sentence:

• Dün = time
• parkta = place
• eski bir arkadaşımla = the person involved
• karşılaştım = verb

So this order is very natural in Turkish. English is much less flexible here, but Turkish can move parts around for emphasis. Even then, the verb often stays at the end.

Parkta is park + -ta, and it means in the park or at the park.

The ending is the locative suffix, which appears as -da / -de / -ta / -te depending on vowel harmony and consonant voicing.

Here:

• park ends in k, a voiceless consonant
• so -da becomes -ta
• the last vowel is a, so the vowel in the suffix is also a back vowel

That gives parkta.

Because Turkish often attaches ile to the end of the noun as -le / -la.

So:

• arkadaşım ile = with my friend
• arkadaşımla = with my friend

These mean the same thing. The attached form is very common in everyday Turkish.

In this sentence, arkadaşımla breaks down as:

• arkadaş = friend
• -ım = my
• -la = with

So literally it is with my friend or, more naturally here, with a friend of mine.

This is a very common Turkish pattern.

Turkish often uses a possessive form where English would simply say a friend of mine or just a friend, depending on context.

So:

• bir arkadaşım = a friend of mine
• eski bir arkadaşım = an old friend of mine
• eski bir arkadaşımla = with an old friend of mine

English often leaves the of mine idea unstated, but Turkish likes to make that relationship explicit.

Here bir works like the English indefinite article a/an.

So eski bir arkadaşım means an old friend of mine.

It can also literally mean one, but in this sentence it is functioning more like an indefinite article.

Compare:

• eski arkadaşımla = with my old friend / with that old friend of mine
• eski bir arkadaşımla = with an old friend of mine

So bir makes it sound less definite and more like one old friend from among your friends.

Because karşılaşmak does not work like a normal transitive verb taking a direct object.

In Turkish, you typically karşılaşmak biriyle — literally, to encounter with someone.

So the person you meet is marked with ile / -le / -la, not with the accusative.

That is why the sentence uses:

• eski bir arkadaşımla karşılaştım

not

• eski bir arkadaşımı karşılaştım — which is incorrect

This is something you usually just learn together with the verb:
birisiyle karşılaşmak = to run into / encounter someone

Karşılaştım can be broken down like this:

• karşılaş- = meet, encounter, run into
• -tı = past tense
• -m = I

So it means I encountered / I met / I ran into.

The past tense appears as -tı rather than -dı because the verb stem ends in ş, which is voiceless. Turkish changes the consonant in the past tense suffix to match that.

So:

• karşılaş + dı + m does not happen
• karşılaş + tı + m is the correct form

Not necessarily. In this kind of phrase, eski usually means old in the sense of longtime, former, or from the past, not elderly.

So eski bir arkadaşım usually means something like:

• an old friend of mine
• a friend I’ve known from before
• a former friend, depending on context

If you specifically wanted to say the person was old in age, you would more likely use yaşlı:

• yaşlı bir arkadaş = an elderly friend

This is an important distinction.

• karşılaşmak = to run into, encounter, meet by chance
• karşılamak = to meet, welcome, receive

So in this sentence, karşılaştım suggests an accidental or unplanned meeting.

For example:

• Dün parkta eski bir arkadaşımla karşılaştım = I ran into an old friend in the park yesterday.
• Misafirleri havaalanında karşıladım = I welcomed/met the guests at the airport.

So even though both can relate to meeting someone, they are not interchangeable.