I was told that Turkish did not really differentiate the meaning of these. It is hard to belive because there a lot of different structures to express a need or obligation in Turkish too. Is it really so?

well, as far I know it is not exaclty like that
for example -mali/meli is something like must or should
Hastayim, doktora gitmeliyim.
I'm ill, I must go to a doctor.
but you can also say:
Cok hastasin, mutlaka doktora gitmen gerek/lazim.
You are very ill, you have to go to a doctor.
or you can also use: gitmek mecburiyetinde olmak, gitmeye mecbur olmak or gitmek zorunda olmak/kalmak which is, as far I know, the strongest
the differences in meaning among them are very delicate, so I think some natie speaker shoul explain it, because I'm confused about using them sometimes.
I think use of them is more intuition than gramatical rule.

Have you checked out this page already?

so, can somebody explain to me:
is mecbutiyetinde olmak or mecbur olmak more like gerek/gerekiyor or more like lazim?
Actually I thought that gerek and lazim is exchangable...
is that true that zorunda kalmak is the strongest among them?
If so, is it something like have to?

"Gerek" and "Lazım" are synonyms.
Actually "Gerek" is a pure Turkish word while "Lazım" is originally Arabic ; but they both mean "necessary","needed","required"...

About must, should, have to, need to / mutlaka , -meli/-malı , gerek/lazım , ihtiyacı olmak

Zorunda olmak/mecbur olmak(mecbur is also originally Arabic) , these two combinations are synonyms too.And meaning "must"
Gerek/Lazım is more like "have to"
-meli/-malı is like "should"
(bir şeyi yapmaya) ihtiyacı olmak is like "need to (do sth)"

In Turkish if you are saying someone that "I must see a doctor" you actually mean "As far as i'm concerned/i think/i belive i should better see a doctor" if this originally is your thought about your situation , but not an order.
->Doktora gitmeliyim/doktora gitmem lazım/gerek.
This is because you feel and believe that way.(If it's sth up to you even when you say "i must" this still would mean sth like "i should" in Turkish)

However suppose a kid's father gave a decision and will take him to doctor tomorrow. Then he should better use "I have to/must see a doctor tomorrow , so i will not come with you" because he was asked to do so , he has no other choice...
->Yarın doktora gitmek zorundayım / gitmem lazım/gerek ; dolayısıyla seninle gel(e)meyeceğim.

This is also true if your situation is critical , suppose you would be tetanos if you wouldn't go to see a doctor.In such situation using "i should" wouldn't be enough...You should say for instance"i must go to see a doctor at once"
->Derhal doktora gitmek zorundayım / gitmem gerek/lazım.

A weird sounding for foreigners : Mutlaka doktora gitmelisin.
(Literally meaning sth like "You should must see a doctor")
This is actually how you would say a close friend if you see she/he's really sick : You must see a doctor.
(You must see a doctor literally means "Doktora gitmek zorundasın")

But in Turkish when you ask or say sth to someone using "must" it would be an order sentence.(Therefore it may be a little rude to use it in many conditions)
Forexample : You mustn't smoke.
->Sigara içmeyiniz.(içmemek zorundasınız)
There's no other option , you have to do what's said.
You can't give order to someone when suggesting sth about their lives as a friend in Turkish. But you may use it If the one you are talking to is your child and you want to make him/her learn sth or obey for instance...

Biraz hava almaya ihticayım var.
->I need to breath some fresh air.
You see there's use for "ihtiyacı olmak" in Turkish but it's not so common.For instance in most situations you would be able to substitute it with such a use : Biraz hava almalıyım.

I know it is not very clear to understand and it's a feeling which you will gain with experience...

Thanks KIVILCIM, I added this to my bookmarks!

Elisa

Gözde, a friend of Kadir, offered that she (and the other friends) can help me when i come to Turkey to live next year.

»» Biz sana yardımcı olacagız

I wanted to answer..

"Hehe i will need it!"

How do you use the word 'need' in this case?


I simplified it and answered her:

"çok teşekkürler, yardimci ihtiyacim var "



But how to say: I will need it???

i need water
->Suya ihtiyacım var.

i need to breath some fresh air.
->Biraz (taze) hava almaya ihtiyacım var.

>hehe i will need it!
hehe buna ihtiyacım olacak!

i will need someone to help me
->(bana)Yardımcı olacak birine ihtiyacım olacak

i will need some help
->Biraz yardıma ihtiyacım olacak

i need help
->Yardıma ihtiyacım var

i will need it/this
->ona/buna ihtiyacım olacak

And that's all from me for today...

Thank you very much KIVILCIM for the explanation!

Greetings,
I have written on this issue before. The text below is a copy of a previous post of mine. Please notice the change on meanings of English words when they are negative (the difference between must and must not etc.)
http://www.turkishclass.com/forumTitle_6_2058

That's a very good question which could be a topic of a lesson if somebody would have write it.

1. must > zorunda olmak (zorundasın, zorundayım etc)
I must go. > Gitmek zorundayım
You must stay here. > Burada durmak zorundasın.

The negative changes the meaning completely and tells us that something is forbidden.
must not > -me+meli (koşmamalı, gitmemeli, yapmamalı Here -me is the negative while -meli expresses neccessity.
You must not move. > Haraket etmemelisin.

2. have to > a. zorunda olmak b. gerekli olmak
You have to go. >
a. Gitmek zorundasın. (You must go)
b. Gitmen gerekiyor. (You need to go)

I can't find a better translation for have to. So either I translate it as must to or need to when affirmative.

The negative can be build same way.
You don't have to go. >
a. Gitmek zorunda değilsin. (You don't have to go. Obviously this can't be translated with must not and vice versa)
b. Gitmen gerekmiyor. (You don't need to go)

3. need to > gerekli olmak
You need to go. > Gitmen gerekiyor.
I need to go. > Gitmem gerekiyor.

Negative can be build the same way:
You don't need to go. > Gitmen gerekmiyor.
I don't need to go. > Gitmem gerekmiyor.

4. should
Depends on the meaning of English sentence. Has different usages in English thus varies in translation.

You should go. >
a. Gitmelisin (present)
b. Gitsen iyi olur. (present)
c. gitmeliydin (unreal past, now ,its too late to go)

You shouldn't go.
a. gitmemelisin (present)
b. gitmemeliydin. (unreal past: You shouldn't have gone)

Here notice that "gitmeliydin" and "gitmemeliydin" are unreal past expression where it is too late to act correctly now. Normally in English perfect temses would fit better in this situation but I think people tend to use present tense as well. For instance while "You shouldn't have done this" is the correct sentence for unreal past some may use "You shouldn't do this", thus I have included the unreal past meanings of should and shouldn't as and

5. may
I prefer to translate this with possibility expressions and the suffix is -ebil

You may go. > Gidebilirsin/Gidebilirsiniz.
You may sit here. > Burada oturabilirsin/oturabilirsiniz.
She might come. > Gelebilir.

The negative changes the meaning completely as in English "may not" is used like a polite way of "must not" if I understand correctly. Again if I understand correcty "may not" and "might not" have very different usages in English. I have seen "may not" used as warnings in the context of "Not possible to...", while "might not" sound to me more like "Possible not to...".


You may not enter. >
a. İçeri giremezsiniz. (It is not possible for you to enter)
b. İçeri girmeyiniz. (Please don't enter)

She might not come. > Gelmeyebilir. (There is a possibility that she is not going to come.)

You might not come. > Gelmeyebilirsin. (There is a possibility that you are not going to come.)

Notice that I'm using "may not" as the unpossibility to do something and "might not" as the possibility for something not to be done.

6. Shall
Shall we go? Gidelim mi?


All in all this is an advanced issue. I would suggest starting to practice with -meli -malı (nessassity suffixes).
gitmek > git > git+meli > git+meli+yim
to go > go > need to go > I need to go.

gitmeliyim > I need to go.
gitmelisin > You need to go.

But the negative with the netaive suffix -me and -ma (added like, me+meli ma+malı has a different meaning.

gitmemeliyim > I must not go.
gitmemelisin > You must not go.

The biggest problem is that, in comparizon, not every version is the same. When I listen to Turks talking, they use those forms as they want to and every time as it is written in grammar...
so zorunda olmak means have to but also must, but gerek has the same meaning... is one of them stronger?

"zorunda olmak" is the strongest. Above I have translated must with only "zorunda olmak".

must < > zorunda olmak

have to < > a. zorunda olmak b. gerekli olmak

need to < > gerekli olmak

"zorunda kalmak" is different than "zorunda olmak" because it expresses unwillingness. When you say "zorunda kaldım" you mean that there was no other choice so you had to do something.

Erdinc! Thank you very much!