You don't. at least not if they are undamaged and properly put on.
NO you cannot
Yes. However you decrease the chances by taking your birth control correctly and by using condoms.
yes, it lessons the chances obviously but the only 100% way to not get pregnant is abstinence
yes, you can get pregnant any time there isn't any 100% way not unless you get your tubes tied or have a hysterectomy. also, condoms are more likely to tear if you use more than one at a time...
By not having sex.
Get on birth control if you're still worried about chances of getting pregnant use condoms while using birth control also.
Not likely at all.
Yes, condoms are not 100% effective. Read the label on the box.
Yes, if it leaked inside the vagina you could get pregnant. Using condoms with spermicide will cut some of this risk.
Yes, condoms can tear.
If a guy comes inside the girl, she can get pregnant no matter what, except if they are using contraception [condoms, etc.]
Yes. You should be using condoms or an effective birth control method if you do not wish to become pregnant.