Which one is correct? 1) I chose this discipline not only because of my passion for saving the environment, but I also find it interesting to work within a complex interface 2) Not only did I choose this discipline because of my passion for saving the environment, but I also find it interesting to work within a complex interface AND one more question: I want to say that I chose (past simple) this discipline, but I find (present simple) it interesting... Or must I use one tense? I've corrected the sentences - either are valid. As to the question about which tense - I would say past simple is fine in both cases ... I'm not a professional language teacher though! Do you mean but or do you mean because in the last sentence? I don't know what you mean by 'complex interface'? The russian translation could also mean 'difficult interface' .. but this doesn't help me! The word 'interface' doesn't have any meaning in this context.

Elizabeth Bales, Thank you so much!!! You've helped me a lot!

From your choices, (2) is better than (1). However, if you want to use "chose/find," I would phrase it like this, with changes in caps: "Not only did I choose this discipline because of my passion in saving THE environment BUT ALSO BECAUSE I FIND it interesting to work within a complex interface." In particular, note the addition of "the" and the deletion of the comma. (I am an editor for a major scientific journal and have a Master's degree in Linguistics with a focus on syntax [i.e., grammar], so I feel confident in these recommendations to you.)