If a modal auxiliary in the direct speech is already in the past form, then the same form remains in the indirect speech:
'You shouldn't smoke in the bedroom,' he told them.
~ He told them that they shouldn't smoke in the bedroom.
'We could be wrong,' I told them.
~ I told them that we could be wrong.
'I could speak Spanish when I was young,' I said.
~ I said that I could speak Spanish when I was young.
'When I was in college I would study till two or three in the morning,' she recalled.
~ She recalled that when she was/had been in college she would study till two or three in the morning.
In the last two examples (with could and would), the modal auxiliaries have past time reference in the direct speech, and therefore backshift entails changing to the perfective.
Czy możecie mi wytłumaczyć proszę co mieli na myśli w tym wytłuszczonym fragmencie? Czy chodzi o to, że nie stosuje sie 'could have spoken' i 'would have studied'?