Someone plays Pact of Negation on my turn and has 5 lands. On their upkeep I cast Word of Command in response of the pact trigger. Can I make him play an artifact if that is the only valid play he could make at that point (not enough mana for the rest)?
EDIT: Play an artifact from hand and not an activated ability.
Yes, what you describe is legal. If your opponent has any spell in hand that they have the mana to cast, you can force them to cast it, and to spend mana on casting it. If this leaves him without enough mana to pay for the pact trigger, this can cause him to lose the game.
Does the artifact have Flash? If so, then yes, you can do that. If not, then no. The card being cast still has to be legal.
Actually, regular timing rules are ignored. Word of command instructs the player to play the card as part of its effect, which overrides normal timing restrictions (it has to - players wouldn't be able to play any card otherwise because normally a player would at least have to have priority in order to be able to do so.)
The clause that the player plays the card "if able" addresses cases where other rules or abilities explicitly specify that the player can't play the card, e.g. you can't use Word of Command to make a player play a land card when it isn't their turn, or cast Surprise Deployment during their upkeep, or cast Doom Blade when there are no nonblack creatures.