6. I (forget) ________ to turn off the stove after dinner. I (forget) ________ to turn off the stove a lot of times in my lifetime.

In the first blank we need the past simple form because of the phrase 'after dinner' we use the past simple which is 'forgot'. For the second blank, the phrase 'in my lifetime' indicates the present perfect tense which is 'have forgotten'. Therefore the answer is: 6. I forgot to turn off the stove after dinner. I have forgotten to turn off the stove a lot of times in my lifetime.