Do I HAVE to use accusative seni instead of "sen"?
In this sentence, yes.
If so, should I treat the Turkish accusative case the same as the German accusative case?
No, because the Turkish accusative case is only used for definite direct objects; the German accusative case is used for all direct objects.
The two are similar but not the same.
Bir elma yiyorum. - I am eating an apple. ("elma" is nominative because it is indefinite. It cannot be accusative in this sentence.)
Elmayı yiyorum. - I am eating the apple. ("elma" is accusative because it is definite.)
Ich esse einen Apfel. ("einen" is accusative)
Ich esse den Apfel. ("den" is accusative)
Oh, I asked that question in another phrase I saw earlier (if I see it I'll delete it) and that's really interesting. It reminds me a lot of french, "Je t'aime" is "I love you" and "Je t'aime bien" (Literally: I like/love you well) is "I like you". The verb "aimer" by itself can mean both "to love" and "to like" depending of the context, and if you use it on a person it's always the former unless you add "bien".
Not in English.
Such verbs of sensation ("love", "feel", "hear", "see", "think") are usually in the simple present even when you are talking about an ongoing action.
At least in US and UK English. You will sometimes hear them in the present continuous in English from India ("I am not knowing that", etc.), but I would recommend that you not use this form.
The accusative goes with definite direct objects -- but "the" is not the only way to make something definite.
Names of individuals are always definite, for example, as are pronouns.
There's only one "you" -- just as "the you" doesn't make sense, neither does the indefinite "a you".
Thanks for your reply. I know that pronouns are definite that's why i was confused as to why the use of accusative. But something you wrote made me understand why i was confused in the first place. 'The accusative goes with definite direct objects', I've always known this rule at the back of my head what confused me is that i thought of 'sen' as the subject not the object. 'Ben Seni seviyorum' SOV. Teşekkür ederim