Are these OK?

1. They went to Phoenix last weekend.

Geçen hafta sonu Phoenix´e gittiler.

2. They will return next Tuesday.

Sonraki salı dönecekler.

3. Turkish ice cream is the best.

Türk dondurma en iyi.

4. I bought new tires.

Yeni lastikleri aldım.

I don´t really understand 3) and 4). Any links to explain would be appreciated. I suspect 4) has something to do with plural. In English we can´t say ´I bought tire´. But we can say ´I bought tires´. Is this only for certain verbs?

3. Turkish ice cream is the best.

Türk dondurması en iyisi.

a)Türk dondurması is an undefined noun clause. That means the second word has to have a suffix. Here are other examples: kapı kolu, çöp kutusu, tren istasyonu, otobüs durağı.

b) It is "en iyisi" because "en iyisi" is a part of a noun clause too. "Tüm dondurmaların en iyisi" You see? But we omitted "tüm dondurmaların" because it is understood without saying it.

4. I bought new tires.

Yeni lastikler aldım.

It is not about being plural or not. If you say "lastikleri" you make "lastikler" defined. It is the same thing with saying "the tires" in English. It would be correct if the sentence was: I bought the new tires. But there is no "the" in your sentence so there is no need to make "lastikler" defined. The singular of your sentence would be like this: "Yeni lastik aldım."