Not exactly. The right way to say both of those sentences would be: 1)"If I don't come back, please switch off my computer" 2)"If you have time, please come and see me."

"If I did not came back" is a hypothetical scenario using simple past tense, so you can combine it with a simple future or conditional sentence: "if she told you the answer, you would be lucky" "if she told you the answer, you will be lucky" But you cannot combine it with an imperative (please, do this or that). If you want to use an imperative, you have to use the first sentences in present. If she tells you the answer, please, come and tell me. If she does not tell you the answer, call my lawyer. The second sentence: "if you had time, please come and see me" is correct. The structure there is a little different though. That structure is originally in something called "subjunctive" that is now almost completely lost in English, with the exception of a few sentences that still remain in use). So, you use the past perfect to create the hypothetical scenario and then the imperative sentence. I hope this helps.