Thank you so much, gokuyum. Expressing past certainty seems to be today´s lesson for me. A couple of more remarks if you don´t mind:

1. Hep “Ağzı Ağız açık ken yemek yenmemeli”: In the English translation there was talk about ´one´s mouth´, ´one´s elbows´. It´s good to know impersonal passive in Turkish works in this kind of instructions which are given to people in general, but it seems that in this case you cannot use 3rd person possessive suffix to refer to this general person, isn´t it?

2. Herhangi birimiz olabilir: Queueing pronouns like this was something new to me but while making the translation I already doubted that the whole meaning was not in birimiz. Interesting.

3. Isn´t there any way in Turkish to express ´by six o´clock´, ´any time before six o´clock but not later´? I thought about saat altıya kadar but it didn´t sound right.

I guess I can´t get further with these on my own:

1. Frank´s wallet is lying on the coffee table. He must have left it here last night. Frank’in cüzdanı kahve masasında yatıyor. Dün gece onu burada bırakmış olmalıydı.

2. Mrs. Scarlett´s body was found in the lounge just moments ago, and it´s still warm! Nobody has left the mansion this evening, so the killer must be someone in this room. It may be any one of us. Bayan Scarlett’in cesedi sadece birkaç an önce bulundu, daha ılıktır. Hiç kimse bu gece konaktan ayrılmadı, o yüzden katil bu odadaki biri olmalı. Birimiz olabilir.

3. Ted´s flight from Amsterdam took more than 11 hours. He must be exhausted after such a long flight. He should prefer to stay in tonight and get some rest. Ted’in Amsterdam’dan uçuşu 11 saatten daha sürdü. O kadar uzun uçuştan sonra çok yorgun olmalı. Bu gece içinde kalıp biraz dinlense ona iyi olur.

4. My uncle’s car broke down last weekend and he had to have it towed more than fifty kilometres. It must have cost a fortune. Amcamin arabası geçen haftasonu kırmış ve o 50 kilometreden daha onu çektirmek zorunda kalmış. Bir servete mal olmuş olmalıydı.

5. When I was a child my grandmother was continually correcting our manners. She always used to say, "One must not eat with one’s mouth open.” Or she would correct us by saying “One shouldn’t rest one’s elbows on the table. And every time I wanted to leave the table, she would say: “One must ask to be excused.” Ben çocukken büyükannem sürekli bir şekilde tavırlarımızı düzeltiyordu. Hep “Ağzı açıkken yemek yenmemeli” derdi. Ya da “Dirsekleri masadan destek alınmamalı” diye bizi doğrulttu. Ve her zaman ben masadan ayrılmak isterken “İzin verilmeli” derdi.

6. I´ve redone this math problem at least twenty times, but my answer is wrong according to the answer key. The answer in the book must be wrong. Bu matematik sorununu en az yırmı kez yeniden çözdüm, ama cevabım cetvele göre yanlıştır. Kitaptaki cevap yanlış olmalıdır.

7. The television isn´t working. It must have damaged during the move. Televizyon çalışmıyor. Taşınma boyunca zarar vermiş olmalıydi.

8. You must be kidding! That can´t be true. Elbette dalga geçiyorsun. Gerçek olamıyor.

9. Firefighters must be able to climb high ladders, swim 400 metres in less than 12 minutes and carry heavy equipment. İtfaiyeciler, yüksek merdivenlere tırmanıp 400 metreyi 12’den daha az dakikada yüzüp ağır bir donatım taşıyabilmeliler.

10. Do you always have to say the first thing that pops into your head. Couldn’t you think once in a while before you speak? Başına gelen birinci şeyi hep söylemeli misin? Konuşmadan önce arada bir düşünebilir misin?

11. Terry and Frank said they would come over right after work. They should be here by 6:00. Terry ile Frank hemen işten sonra gelecekleri söylediler. Burada saat altıda olmalılar.

12. Israel may be forced to attack Iran’s nuclear facilities. İsrail İran nükleer tesislerine saldırmak mecburiyetinde kalabilir.

Thank you for posting this, harp00n.

But that´s what I was looking for, si++. I was asking if değil also will do "if we want to negate the necessity only" which makes the meaning of ´not necessary, not obliged´. I mean -memeli cannot mean ´not necessary´ but maybe -meli değil can (just like gerekmiyor or lazım değil).

I guess I understood why -memeli cannot mean ´doesn´t have to´. In Turkish modally modified verbs the negation marking is attached to the morpheme which is in fact denied. It is attached after it, not before it. Thus the negating morpheme -me- can only concern the verb root, not the necessitative -meli-. So, etmemeli has no other choice but to make ´it is necessary that he doesn´t do´. If we want to negate the necessitative only we have to use roundabout expressions like lazım as mentioned before in this thread.

What about değil? It is the usual way to negate an adjective. I even found occurances:

         Gitmeli değil miydik?

         Onunla birlikte melekler gelmeli değil miydi?

You know, I must have thought of the opposite but mixed it while writing...sorry.

______________________

Forms like et|me|yebil|ir ´may not do´ or koş|ama|yabil|ir ´may not be able to run´ really demonstrate the simple logics of Turkish morphology. Look at the place of the negation: it is attached to the part which is in fact denied, not to a grammatical hanger like in auxiliary languages. This is a very pedantic way to express the meaning. Actually you should translate them ´it may be that he doesn´t do´, ´it may be that he is not able to run´.

These epistemic examples also show that -(e)bil- still very much represents a full lexical verb even though due to agglutination it is usually seen as a grammatical morpheme.

Just a couple of modest notes, nothing important.

This pregnant thing is an old and worn out joke but what I can´t resist is

         Sen benim herşetunsin.

I am laughing here by myself. I even memorized it.

For sure these things have been collected many times somewhere. It looks like a job that a professor could give to a seminar student. But how to put it in a simple way for a learner and  -  more than that  -  as a learner is another question. There are so many factors to take into account.

Is ´he may be able to run´ koşabilebilir?

Morphologically, -(e)bil- can come after -meli- but not vice versa, is it?