I heard that "xy sevdim" is actually used for "I love xy" in the present. The esplanation was something like this: I started loving it in the past, and I still love it now. If I would say "xy seviyorum" it would mean, that I love it only just right now, in this moment, but usually I don't. What is the experts' opinion on this? Sağ ol!
actually I don't agree :) "sevdim" for "I like/love" is rather like a reaction. You have a look at it and you have an idea ---you use present in English and past in Turkish. If I see your new shirt, I would say "Gömleğin ne güzel, çok sevdim!" (How beautiful is your shirt, I love it!). But I would rather say "Sarı gömleğini çok seviyorum, bugün onu giy" (I like your yellow shirt a lot, wear it today). So as you can see "seviyorum" is actually more general, "usually" I like it, I have liked it so far and I still do.
I remember hearing the phrase "seni öptüm!", which i believe you would translate with a present meaning in English ("I kiss you") and many other languages. So, to me, "bu kitabı sevdim" was reinforcing the idea that Turkish likes to use the past in some situations when the present is used in other languages. I have trouble using your comment to explain the "seni öptüm" phrase.. May I ask what your thoughts are about "seni öptüm"?
In Turkish, if you are stating a reaction to something "saying that you like/love something that already happened), you use the past tense. The present tense is reserved for general statements.
If you want to say that you liked something in the past, you use the past perfect (sevmiştim/beğenmiştim/etc.)
In English "I liked" does not imply that I still like it - I may and just as well I may not. (To me it rather has a slight connotation of the latter, but maybe that is just me, any native English speaker please correct me if I am wrong). Which brings me to the question how one would express that "I liked" in Turkish? With a different tense?
The first one would be "I had liked." It is used for things that occurred in the distant past and in some clauses.
The second one is the reported past tense. It means that it happened in the past without you directly witnessing it (and it is strange to use in the first person in most, but no all, contexts).