7. The children (hide) ________ in the basement yesterday. They (hide) ________ in the basement often since they (discover) ________ a secret place there.

The first blank requires past simple, so we use 'hid' for the past tense of hide. The second blank is a present perfect because of 'since', therefore we use 'have hidden'. The third blank also requires past simple, because its an action in the past when they found the place, so the past tense of discover is 'discovered'. Therefore, the answer is: 7. The children hid in the basement yesterday. They have hidden in the basement often since they discovered a secret place there.