Steel sinks when it into water. (put) The soldiers parcels to the army. ((often) send)
Milk goes sour when it for three days. (left)
She was very sad because her ring . (found)
The delegation at the station in an hour. (met)
They helped him and everything in time. (done)

Steel sinks when it is put into water.  (Это научный факт, используем Present Simple и страдательный (пассивный) залог, т.к. сталь помещается (кем-то) в воду)

The soldiers are often sent parcels in the army. (Регулярное действие в настоящем Present Simple Passive. Пишу в настоящем времени, раз нет обстоятельства времени - отсылки к )

Milk goes sour when it is left for three days. (тоже факт)

She was very sad because her ring wasn't found. (Она была очень опечаленной, потому что ее кольцо не было найдено. - Past Simple Passive)

The delegation will be met at the station in an hour. (Делегация будет встречена на станции/вокзале через час. - Future Simple Passive)

They helped him and everything was done in time. (Они ему, и всё было сделано в срок. - Past Simple Passive)