Why do you sabotage me scalpel?

Yes, Marion, it is certainly wrong but the language community doesn´t always agree about everything and it seems to me that it is a very usual misunderstanding. (Psst, *gerekiyorum often takes noun object: *Yeni bir fermuar bu palto için gerekiyorum. It looks more like a synonyme to ihtiyaç olmak.)

Just helping you  

Thank you

-en suffix is free from "time":

İşte almam gereken çiçekler bunlar. (present)

Torbasını açması gereken yolcu telaşlanacaktır. (future)

Kızının kasapla evlenmesi gereken baba üzüntüsünden kahrolmuştu. (past)

Söylemem gereken şeyler olduğunu düşünüyor. (present continuous)

Vermesi gereken parayı geciktiriyordu. (past continuous)  

Vermesi gereken parayı geciktirmesin. (imperative)

Vermesi gereken parayı geciktirirse kötü olur. (conditional)  

I believe you, scalpel, but aren´t the sentences ambiguous? What if the necessity was long gone and the main clause action is still to come?

A nice set of examples. I will copy-paste and hide it to my personal files.

Keep believing any native speaker of Turkish.. This "native speaker" either be me or gokuyum or someone else..   No, they are not ambiguous.. Why would they be?. Because of the -en suffix?. It saves you from tenses and yet you don´t like it  Even when "the necessity was long gone and the main clause action is still to come", you can use gereken safely: 

üç yıl önce ödemesi gereken borcu önümüzdeki ay ödeyecek.

As gereken is an adjective (participle ), it´s only mission is to modify the following noun.. The problem is that in this type of sentences gerek- doesn´t take -dik suffix.. But to solve the problem,if there is any, instead of gerek-, you can use zorunda ol- that, to the contrary of gerek-, it takes -dik suffix but not -en in this type of sentences:   

üç yıl önce ödemek zorunda olduğu borcu önümüzdeki ay ödeyecek.

They have the same meaning and I think the former is preferred ..

When I used the word ambiguous I didn´t mean there is something wrong with the sentence. Minimalism is the strategy of Turkish language and there are always lexical ways to make the utterance more exact when necessary (like you added üç yıl önce above).

I have a strong trust for natives, scalpel. If the first native gives me an answer that I doubt I can always ask another native... Seriously, I keep asking things only because I can´t keep in mind things that I don´t understand.

Same here!. I really do understand you.. you should keep asking till you get the answer you are looking for..  

How can I express a subclause necessity in the past when the main clause is in present tense and the subclause is

a) an interrogative content clause (indirect question)?

“Sometimes she wonders if she should have left a long time ago.”

b) a declarative content clause?

“Sometimes she thinks she should have left a long time ago.”

I am afraid it has to do with gerekmek again.