Translation - Turkish-English - ben eski konuları konuşmak istemiyorum.o günler...

Hi Mesud

I have made some edits; do let me know if you don't agree.
Here is your translation before my edits:

I don't want to talk about old subjects. Those days are left behind. Let's concern about the future.

Hi yourself Lein

Your edits are fine with me. But I wonder if my translation sounds good. (or your edits are more common)

'Those days are gone' is the usual way of saying this. Technically, 'left behind' is not wrong, but it is not used here.
Let's concern about - 'concern' is a reflexive verb (I think that is the correct term in English!) which means it needs 'ourselves', 'myself' etc to follow it. Alternatively, you could use 'concerned', as in 'I am not concerned about this'.

But in your sentence, there was definitely something missing!

'concern about' (or, perhaps more common, 'to be concerned about') means to worry about something.
'concern with' is to give attention to, which I think is the meaning here.

Yes, you're right. Thank you