The last is nearly grammatical (needs had rather than had, and delete the before half), but unlikely to be said except in a formal or official context.

The third ("halfway") sounds all right in context (but when I saw the title of the question I wondered what on earth it meant).

The way I would say it is

When I arrived at the warehouse , they had already half loaded the truck.

The last is nearly grammatical (needs had rather than had, and delete the before half), but unlikely to be said except in a formal or official context.

The third ("halfway"), if similarly amended, sounds all right in context (but when I saw the title of the question I wondered what on earth it meant).

The way I would say it is

When I arrived at the warehouse , they had already half loaded the truck.

The last is grammatical, but unlikely to be said except in a formal or official context.

The third ("halfway") sounds all right in context (but when I saw the title of the question I wondered what on earth it meant).

The way I would say it is

When I arrived at the warehouse , they had already half loaded the truck.

The last is nearly grammatical (needs had rather than had, and delete the before half), but unlikely to be said except in a formal or official context.

The third ("halfway") sounds all right in context (but when I saw the title of the question I wondered what on earth it meant).

The way I would say it is

When I arrived at the warehouse , they had already half loaded the truck.