Main Phase 1

Main Phase 1
The Main Phase 1 is the Main Phase conducted after the Standby Phase.
Most actions that could normally be performed in Main Phase 1 can also be performed in Main Phase 2.

After Main Phase 1
After your Main Phase 1 has ended, you may choose to enter the Battle Phase. If you choose not to, you automatically proceed to the End Phase (you cannot enter Main Phase 2 unless you conduct your Battle Phase).

Only during Main Phase 1
Some Cards and Effects can only be activated during Main Phase 1, such as "Cold Wave". Many of these Cards possess this restriction because of their Effect during the Battle Phase, such as "After the Struggle".

In Main Phase 1, you may:
Normal Summon/Set a Monster (you can only perform one Normal Summon/Set per turn, so you cannot Normal Summon/Set in both Main Phases).
Special Summon a Monster(s).
Manually change the battle position of a Monster, provided that Monster was not Summoned or Set during that same turn, and it has not already manually changed its battle position that turn.
Flip Summon a Monster, provided it was not Summoned or Set during that same turn.
Activate Ignition Effects, such as those of "Barrel Dragon".
Set Spell/Trap Cards.
Activate Spell/Trap Cards of any Spell Speed. You cannot activate Quick-Play Spell Cards and Trap Cards the turn you Set them.