Spanish speakers, i want to know the truth.?

ok if you've grown up naturally speaking spanish, do you really use all those grammar rules? like ok there are 2 past tenses in spanish the preterite and imperfect. preterite is a completed past action. when you speak do you realy have to think about do i use the imperfetc or preterite? i mean we'll still know you're talking in the past, but it just won't be grammatically correct. and the subjunctive is used to express doubt. when you speak about doubt do you worrying about using the subjunctive, or just say it in the present tense. im just wondering. im in spanish 3.