Was to and have to. Was to is sometimes used to explain something that was supposed to happen but could not or did not happen. For example:" The man was to go to work today but he fell ill during the night" or " The man was supposed to go to work this morning but he fell ill last night: Have to is used when something is still expected to happen. For example" The boys have to take the train to school tomorrow" or " The girls have to take the taxi to school later today" Good luck with your English!! Teacher Hannes

"The group was to remain there until..." would be the past tense of the sentence. "The group has to remain there until..." would be the present tense of the sentence. "The group" is a singular subject referring to a specific group therefore requiring a singular verb tense (has to versus have to).