I can't find the steward In Morthal and the Jarl won't send me on quests.
I have Hearthfire and a house in Whiterun and Falkreath. I am thane in both.
Do I have to be thane in Morthal as well?
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Try talking to the Steward during business hours (8AM-8PM). He should be in the halls or somewhere in Morthal. Yes, you must be a Thane in Morthal in order to purchase a house.
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