Concord of tenses is if the main verb is in a past tense, the verbs in the subordinate clause expressing simultaneous activity must be in a past tense as well.
It was raining when I come to the train station last night.
They lived in a house which had red walls.
She drank coffee which had strong taste.
He walked all day , so he tired himself.
The Verb may remain in the simple present tense if in the subordinate clause expresses a natural law or an eternal truth, or a habitual fact or activity.
She wanted to know why I run every day.
I was told that you are a basketball player.
We were glad to hear that you are all right now.
If the main verb is in a past tense, the following verbs, expressing an activity precedent that of the main verb, must be in the past perfect tense.
I thought I had seen him.
They went home after they had finished their work.
It was the first time that i had been there.