There are rules in converting direct to indirect speech. The tense usually has change because when we use reported speech, we are usually talking about a time in the past (obviously the person who spoke originally spoke in the past).
1. Tense Change ; As a rule when we report something someone has said you go back a tense (the tense on the left changes to the tense on the right)(an we can use the Present Tense in indirect speech if we want to say that something is still true.)
2.Modal Change ; Modal verb forms also sometimes changeIf it use modal, such as: could, would, should, might and ought to in direct speech, then we don’t need to change it.
3. Time Change ; If direct speech sentence contains an expression of time, we must change it to fit in with the time.
4. verbs ; Some reporting verbs may appear in more than one of the following groups.
Tense Change , modelchanges and time changes as well.