Clauses indicating when something happens, introduced by words such as ubi (when) or postquam (after) have an indicative verb and are usually straightforward to translate.
ubi and post quam are followed by a perfect tense where in English (and the corresponding *** clause), we would have a pluperfect:
ubi Romam adveni, tabernam quaerebam. When I had arrived in Rome, I started looking for the shop.
BEWARE OF THE HIDDEN FUTURE: English uses a present tense where Latin uses a future tense, or more often a future perfect tense (if the first action preceeds the second):
ubi Romam advenero, imperatorem salutabo. When I arrive in Rome, I shall…