Mark ...in his room,when he...a horrible noise 1) Studied, heard 2)was

Mark was studying in his room, when he heard a horrible noise. Марк обучался в собственной комнате, когда услышал страшный шум.

We use Past Continuous in the first part of the sentence. We use past Indefinite in the second part of the sentence. Mark heard a horrible noise when he was in his room.

Past Continuous is formed with the help of past form of the auxiliary verb to be (was / were). The suffix of past Continuous is ing.

Past Indefinite is formed with the help of the suffix ed for regular verbs and we use the second form of irregular verbs.