How do you balance chemical formulas!

im SOOOO CONFUSED!!!
Could u plz explain it then help me with these?
1. Na+MgF2 -- NaF + Mg
2. Mg + HCl -- MgCl2
3. Cl2 + Kl -- KCl + I2
4. NaCl --Na +Cl2
5. Na + O2 -- Na2O
6. Na + HCl -- H2 + NaCl
7. K + Cl2 -- KCl
Challenge- C2H6 + O2 -- CO2 + H2O
dont worry once i figure out how to do it i will do the rest on my own. there is an entire other side to do!
oh and if you could help with this possible too
atoms are not _____ or _____ duringa chemical reaction scientists know that there muct be the ______number of atoms on each _____of the ________. to balance the chemical equation you must add _______ in front of the chemical formula in th eequation. you cannot drop or change subscripts!

I'm sorry I can't help with the fill-in-the-blanks, but i can balance equations!
The object is to have the same amount of moles of each element on each side. I think of it as a puzzle, and these are simple when you get the hang of it.

1.) First you look for the obvious: on the reactant side flourine has two moles (F2). on the product side there is only one mole of flourine (F). to balance this add a 2 to the compound NaF.
Na+MgF2 => 2NaF + Mg
Then you notice that now you have the same amount of F moles on each side; however, now you have 1 mole of Sodium on the reactant side and two on the product side. You should add a 2 in front of Na.
* 2Na+MgF2 => 2NaF +Mg
Now if you count the moles on each side... 2 Na on each side, 1 Mg on each side, 2 F on each side.
You're completely balanced now so you have your final answer(*).

2.)Is there a hydrogen on the end because you can't balance it if there isn't
Mg+2HCl => MgCl2+2H
3.) Cl2+2KI => 2KCl+I2
4.) 2NaCl => 2Na+Cl2
5.) 4Na+O2 => 2Na2O
6.) 2Na+2HCl => H2+2NaCl
7.) 2K+Cl2 => 2KCl
CHALLENGE:
This one is a little tricky and you can erase and play with the numbers until you get it right.