Here is something on relative clauses:
Modern grammar rejects many of the ideas of the past. the most important area of this is the tense system. Modern grammar suugests that there are onlt two tenses in English and they are not organised by time- time may be a factor, but the concept of proximity or remoteness is the governing force. If we are remote (far from) an action, we use the past tense. This remoteness could be time (past), possibilty (future- would) or social distance. Where we are near an action, we use the present.
It's a very big subject and I could help more if you asked more specific questions about what is a very interesting area of language.