Arkadaşım ve ben hafta sonu kütüphanede buluşmayı kararlaştırdık.

Because arkadaş means friend, and the ending -ım means my.

So:

• arkadaş = friend
• arkadaşım = my friend

The exact form of the possessive ending changes by vowel harmony, but here it becomes -ım.

Both are possible, but they are not exactly the same in emphasis.

• biz = we
• arkadaşım ve ben = my friend and I

Using arkadaşım ve ben makes it clear who we are. It is more specific.

Also, putting ben second is very natural, because Turkish often places the speaker after the other person in coordinated subjects, similar to English my friend and I rather than I and my friend.

Literally, it means week end:

• hafta = week
• son = end

Together, hafta sonu means weekend.

Turkish often keeps this as two words. It functions like a fixed time expression.

In this sentence, hafta sonu is being used as a time expression meaning on the weekend / at the weekend.

Turkish often uses time expressions without an extra case ending:

• yarın = tomorrow
• pazartesi = on Monday
• hafta sonu = on the weekend

So hafta sonu here works adverbially without needing another suffix.

Kütüphane means library.

The ending -de is the locative suffix, meaning in / at / on depending on context.

So:

• kütüphane = library
• kütüphanede = in the library / at the library

In this sentence, it tells you the place where the meeting will happen.

Because the sentence is about the location of the meeting, not movement toward the library.

• kütüphanede buluşmak = to meet at/in the library
• kütüphaneye gitmek = to go to the library

So -de answers where?, while -ye/-ya would answer to where?

Buluşmayı comes from buluşmak, which means to meet.

Here, Turkish turns the verb into a noun-like form:

• buluşmak = to meet
• buluşma = meeting / the act of meeting
• buluşmayı = the meeting / meeting (as a specific object)

Turkish often uses these noun-like verb forms where English uses to + verb.

So instead of something that looks exactly like English to meet, Turkish says something more like the meeting as the object of kararlaştırdık.

No. Here -ma is not negative.

In buluşmayı, -ma is the suffix that turns the verb into a verbal noun/action noun.

• buluşmak = to meet
• buluşma = meeting

If it were negative, the form would be different:

• buluşmamak = not to meet
• buluşmamayı = not meeting / the act of not meeting

So in this sentence, buluşmayı is positive, not negative.

The ending -yı is the accusative case.

It appears because buluşma is the direct object of kararlaştırdık.

So the sentence is treating meeting as the thing that was decided/arranged.

The y is just a buffer consonant. Turkish often inserts y between two vowels:

• buluşma + ı would be awkward
• so it becomes buluşmayı

Kararlaştırdık means we decided / we agreed on / we arranged.

A simple breakdown is:

• kararlaştır- = decide, settle, arrange
• -dı- = past tense
• -k = we

So kararlaştırdık = we decided

Because the subject is Arkadaşım ve ben, the we ending makes sense.

Yes, that would also be natural Turkish, though the structure changes.

Two common patterns are:

• buluşmayı kararlaştırdık = we decided/arranged to meet
• buluşmaya karar verdik = we decided to meet

The difference is mainly grammatical:

• kararlaştırmak takes buluşmayı as its object
• karar vermek uses buluşmaya with the dative ending

Both are good Turkish. Kararlaştırmak can sound a bit more like settled/arranged, while karar vermek is the more straightforward decided.

Because Turkish normally prefers verb-final word order.

A very common Turkish pattern is:

subject + time + place + object + verb

That is exactly what you see here:

• Arkadaşım ve ben = subject
• hafta sonu = time
• kütüphanede = place
• buluşmayı = object
• kararlaştırdık = verb

Turkish word order is flexible, but this sentence follows a very typical pattern.

Because buluşmak already means to meet each other / to come together.

In this sentence, both people are already included in the subject:

• Arkadaşım ve ben = my friend and I

So Turkish does not need an extra with here.

If only one participant were the subject, then Turkish could use ile / -le:

• Arkadaşımla buluştum = I met with my friend

But in your sentence, the two people doing the meeting are already named together, so no extra marker is necessary.