He said that he left/had feft/would leave the office the day before at 6 o’clock.
John said that he would bring/will bring/had brought some flowers to Mary the day before.
Jo says that he had gone/will gone/go on a cruise on Sunday.
David says that he would buy/will buy/had bought a new house soon.
They said that they washed/had washed/will wash the windows the day before.

"He said that he left/had feft/would leave the office the day before at 6 o’clock."

In this sentence, the correct form is "He said that he had left the office the day before at 6 o'clock."

The verb "left" is used in the past simple tense because the action of leaving the office happened before the time of speaking (the day before). The past perfect tense "had left" is used to indicate an action completed before another past event. Since the action of leaving the office happened before the time of speaking, we use the past perfect tense.

"John said that he would bring/will bring/had brought some flowers to Mary the day before."

In this sentence, the correct form is "John said that he would bring some flowers to Mary the day before."

The verb "would bring" is used to express future in the past. John is referring to a future action that he intended to do in the past, which is bringing flowers to Mary.

"Jo says that he had gone/will gone/go on a cruise on Sunday."

In this sentence, the correct form is "Jo says that he will go on a cruise on Sunday."

The verb "will go" is used to express a future action. Jo is talking about a future event that will happen on Sunday, so we use the verb "will go".

"David says that he would buy/will buy/had bought a new house soon."

In this sentence, the correct form is "David says that he will buy a new house soon."

The verb "will buy" is used to express a future action. David is talking about his intention to buy a new house in the future, so we use the verb "will buy".

"They said that they washed/had washed/will wash the windows the day before."

In this sentence, the correct form is "They said that they had washed the windows the day before."

The verb "had washed" is used in the past perfect tense to indicate an action completed before another past event. The action of washing the windows happened before the time of speaking (the day before), so we use the past perfect tense.