The player's inventory is very different from vanilla. The number of slots is initially much smaller. The inventory crafting grid is 3x2 slots. There are two additional special equipment slots, yoke and backpack. These slots allow items which can increase the number of available inventory slots. The offhand slot also has a more specific function for carrying large items which can't be held in any other slots.
Main inventory slots
Initially, only one row of inventory slots is available (the hotbar). More slots become available by using special items, up to a maximum of four rows total (equivalent to vanilla).
The contents of these special equipment slots also affect the player's movement speed.
Backpack and yoke
These equipment slots each accept only their special item. Equipping a backpack adds one extra row to the inventory. Equipping a yoke adds two extra rows to the inventory. These effects are cumulative, i.e. equipping both a backpack and a yoke will provide three extra inventory rows (a total equal to the vanilla inventory).
The inventory of the backpack and yoke is not stored in the item. Unequipping one while there are items in its associated slots will cause those items to move into other available slots automatically, or be dropped if they do not fit. The yoke and backpack can be carried in normal inventory slots, where they do not increase the capacity.
The offhand slot accepts any item. However, some heavy or bulky items can only be carried in the offhand slot, meaning only one such item can be carried at a time.
The crafting grid is 3 columns by 2 rows. The recipes available in the inventory are completely different to vanilla.
Picking up items
There is a new delay function to help dealing with a smaller inventory. Whenever the player drops an item, no other stacks of that item will be added to the inventory until 100 ticks (5 seconds) have passed.