Herkese merhaba! 

I would like to know what is the difference between the two tenses in Turkish. I learned that -iyordu (past continuous) is used in the following cases:

1. talking about actions that last for a long time:

Öğle yemeğinden sonra Ahmet babasına yardım ediyordu.

2. for actions which were in progress at a certain point of the past:

Ayşe dün akşan saat sekizde sinemaya gidiyordu.

3. parallel actions:

Ben kek yaparken Erdal uyuyordu.

4. "when":

Patlamayı duyduğumuz zaman TV seyrediyorduk.

5. saying "all" day, night, week, etc.

Dün bütün gün matematik çalışıyordum.

However, -irdi (used to) is used for past actions that were a kind of habit in the past, but they are not that anymore.

Eskiden kedileri severdi ama kedinin biri ona saldırdı ve o da artık onları sevmiyor.

... Everything was as clear as water so far, but then I started to come across sentences like the following ones:

Eskiden herkes arkadaşına mektup yazıyordu, şimdi e-mail gönderiyor.

Ben lisede her gün ders çalışıyordum, şimdi az çalışıyorum.

I would translate both sentences with "used to", since they are past habits that are not carried out anymore:

Everybody used to write letters to their friends, now they send e-mails.

I used to study every day at secondary school, now I study less.

So, my questions are:

Can -iyordu (past continuous) be used in the same sense as "used to"?

Then how do I know which one I have to use when I want to express "used to"?

Şimdiden çok teşekkür ederim!!  

As a learner I cannot really answer your questions, Lillita, we need a native view for that. I will just share some of my thoughts and some things I have learned from literature (mostly Göksel - Kerslake 2010 ).

I think the correct native-like use of tenses is one of the most difficult things in Turkish. That is because tenses in any language are not only tenses but are also connected to the aspectual and modal views the speaker wants to express.

It is difficult to talk about Turkish tenses without referring to the notion of aspect at the same time. Compare the following sentences to understand the terminology:

She was singing (1) when I entered (2).

The neighbour´s dog used to wake my up every morning by barking. (3)

John is singing. (4)

(1) IMPERFECTIVE, not completed action

(2) PERFECTIVE, completed action

(3) HABITUAL

(4) PROGRESSIVE, action in progress

The Turkish equivalents are roughly as follows:

(1) -(I)YOR, -MAKTA, -(I)R, -(Y)DI (when attached to nominals)

(2) -DI, -MIŞ

(3) -(I)YOR, -MAKTA, -(I)R (in the past), olur/oluyor (in nominal sentences)

(4) -(I)YOR, -MAKTA

For habitual action, both -(I)YOR and -(I)R do the job at least in the past tense. The distribution between them is difficult for a learner to grasp.

The only meaning that -(I)R cannot convey is the progressive aspect. In my opinion, all your examples 1-5 represent progressive aspect and -(I)R cannot be used in them.

As a rule of thumb -(I)YOR partly laps together with the English BE + -ING construction (progressive action) but also has a wider use (habitual action).

If I was a native English speaker which I am not I would compare the use of would and -(IR)DI. Interesting similarities could come into light. Both can be used both for habitual action in the past and unreal (conditional) action.

ANSWERS TO YOUR QUESTIONS:

Can -iyordu (past continuous) be used in the same sense as "used to"?

Yes.

Then how do I know which one I have to use when I want to express "used to"?

You must live Turkish. Expose yourself to it. Watch movies with subtitles, get Turkish friends, read...

I say, with no authority other than being a native speaker of Turkish, "used to" would be "-ardı" That is a combo of simple present and simple past.

"I used to eat my boogers when I was a kid"

"Çocukken sümüğümü yerdim."

Such a short verb might make is difficult to comprehend, so here´s another one:

"I used to drink tea with sugar."

"Eskiden çayı şekerli içerdim."

Yeah another short word. But, I´m not getting paid or anything to reply you. So this is all the examples you´ll get.

Let´s dissect now.

Back in the day, you´d use simple present for something you used to do. That is, if you say "I used to drink tea with sugar", six years ago or something, you´d say "I drink tea with sugar"

Turkish has evolved more logically than English. You add a simple past after the simple present to express "used to":

root: içmek

iç-er-dim

içer = drinks

içdim (due to vocal harmony, it becomes içtim) = I drank

"içerimdim" would be wrong though, as it mentions me or I twice. It would be so wrong noone would understand.

Then again, I would not be content if I do not give additional details, so let´s see another sentence:

"My computer used to make noises."

"Bilgisayarım ses yapıyordu."

Why dafuk did I use "-iyordu."?

Cuz this is Turkish. That´s the way we say it.

"Bilgisayarım ses yapardı" would be "My computer would make noises"

After you go beyond a certain level in a language, you´re lucky to find a one-to-one correspondence of anything. Everything that could be said is a huge surface and different languages cover it with different blankets of different sizes, differently. Furthermore, a correct translation of a correct translation does not always have to come back to itself.

That´s why the answer to your question

Then how do I know which one I have to use when I want to express "used to"?

is "live Turkish".

Anyway, I´ll give you a key hint about "used to". If you want to say "used to" in Turkish, use "eskiden". That would make what you mean a lot clear:

"Eskiden bilgisayarım ses yapardı." See? Even this sounds better.

By the way, you could substitute "eskiden" with other stuff; like "çocukken" = "when I was a kid" or "Ankara´da öğrenciyken" = "when I was a student in Ankara"

But, again, the colloquial conventional way of saying it is "bilgisayarım ses yapıyordu."

"Eskiden bilgisayarım ses yapıyordu." also correct, perfectly understandable. Why one upon another? Depends on the situation. This is the kind of stuff that gives away you´re a nonnative.

It depends on the context, the neighbourhood of this sentence. How? I have some intangible explanations. But, who cares, noone is paying me.

You could pay me though. Not with currency but by helping me learn German. Are you a native German? Then I could explain to you for hours.

It was a pleasure anyway. Being a smarty-pants is always fun.

One more thing,

This is what I said.

But I have one more quality that gives me more authority. I studied my ass off for ÖSS, the Turkish university entrance exam. One part is Turkish. I have years of training in grammar and comprehension and other shite.

Also having tried to learn many other languages gave me a certain amount of insight.

Exactly. And we can go even further: not every language feels the urge to convey the same amount of information about the state of affairs described. There are gaps between the blankets. A simple example is grammatical gender.