Orada havlular var, ama çok temiz değil.

There are towels over there, but they are not very clean (for instance in a hotel)

Thank you

Yes it seems correct, but I don´t know whether it´s better or not to say "çok temiz değiller"

 ..then you don´ t use "havlular"

Thank you Zeytinne....so...both will be too much

 Sentence has two parts. In the second part there is a hidden subject "onlar" When we don´t say it we should use "ler" at the end of the verb to point out the plurality.

By the way there is not such a rule as you can´t use two "ler" in the same santence. Sometimes I prefer to say so sometimes not.

Onlar geldiler. (When you google it you find 6.790 results)

Onlar geldi. (When you google it you find 21.200 results)

Most of the time I prefer the second one.

When the subject is singular, the predicate is singular: o geldi 

When the subject is person+plural, the predicate is plural: onlar geldiler

When the subject is animal/thing+plural,the predicate is singular: onlar geldi

for more information please click the following link

http://www.baktabul.net/turkce/23784-ozne-yuklem-uyumu.html

 Thank god a good explanation. But i googled this and the results are interesting.

"Adamlar geldiler" 2.600 results

"Adamlar geldi"  15.500 results

As you can see people prefer not to obey the rule. Why? Because people always prefer the shortest way to say something. Because of that many words, verbs changed. They became shorter. Also rules can change.

Tamam, google "geri iade" and see the result: 604.000. Just because it is repeated 604.000 times, doesn´t make it any less idiotic, right? Don´t trust such numbers too much.

When possible, preferring the shortest way is what we all do in any language in the world...but we must not allow the language to be reduced back to such a degree as it was 1000000 years ago,right?

A tiny note: I highly recommend you to read her (Feyza Hepçilingirler,the writer of that text) book with the name "Türkçe Dilbilgisi, Öğretenlere ve Öğrenenlere" before talking about the Turkish Grammar.  

"geri iade" can seem you idiotic but it is not. There can be such  semantic shifts in languages. For example serbest means "başı bağlı(dependent)" in Farsi but it is the opposite in Turkish. Also peş means "ön(front)" in Farsi but we use it as "arka(back)". Even if you think  it is idiotic to use them this way you can´t do anything to stop people. Because only people can decide what is proper to use in any language not linguists. If you try to set rules people don´t obey you will create another language; an elite language like Ottoman Turkish. You should accept the language as it is. Language is like a living organism. It grows and changes. Nobody can stop this. Grammer rules also have to change. They are not like ten commandments.