So when it comes to talking about the Mjolnir it get rather confused between the Gen and Mk. and it’s rather frustrating cause I try to watch lore videos but when they start talking about the armors my mind just gets confused and I would like someone to please clarify the armor Gen and Mk. if it’s possible. I know it’s a lot probably nobody is gonna be able to do it in a rather good in depth way, but thanks in advance for those who help me understand better 
Gen 1 Mjolnir includes Mk 1-6.
Master Chief is wearing Mk5 Mjolnir in Halo CE, but he upgrades to Mk6 in Halo 2, which he wears through Halo 3.
Gen 2 armor is the next variant after Mk6, and is what the Spartan 4s wear.
JNDreher is spot on, but I’ll add that Gen 1 Mjolnir actually goes from Mk 1-7. Only known field use of Mk 7 was Naomi-010 though I believe.
But so I don’t get too technical or confusing, I’ll lay it out below:
Gen 1 Mk 1-3: basically advanced exoskeletons developed decades prior to the Spartan-II Program. They required massive amounts of energy and needed to either be directly plugged into a power source or charged frequently, so they weren’t much use on the battlefield. An example of these in use was during the Spartan-II training days on Reach where instructors would wear them in order to protect themselves during training exercises against newly augmented Spartan-IIs
Gen 1 Mk 4-7: these are the armors most people are familiar with. Jerome and Red Team wear Mk 4 during the Halo Wars game. Chief wears Mk 5 during CE and Mk 6 during H2 and H3. As I said above, Mk 7 is pretty rare and has only been used in the field by one Spartan as far as I know (there’s also Mjolnir Black, but that’s an ONI skunk-works prototype only deployed by Black Team).
Gen 2: the UNSC dropped the Mk naming convention in Gen 2. They went for a more modular design so parts and armor pieces could be swapped out pretty easily between designs. Corporate partners started developing their own Gen 2 armors at this point in the program.
Gen 3: don’t know much about this generation, but it looks amazing imo and it’s what the Chief will be wearing in Infinite.
Here’s a Halopedia link to the main page that covers all the armors
Holy mother of Yap Yap thanks guys