Modals and obligation
In the diagram below we can see that both should and must can be used to talk about present or future obligations. Again, when we want to talk about obligations in other tenses we can use other verbs which aren’t modals. The most common one is have to. Use have to in the past and with would, will or other modals (remember we never use two modals together). Review the examples below the diagram and tray to write some examples yourself. Then try to used have to for obligations in other tenses.
He should wait before entering the plane. (not a strong obligation)
They must sit during the takeoff and landing. (strong obligation)
You should get a taxi from the airport after you disembark.
She must file the report before 8pm tomorrow evening.
They had to divert due to weather conditions. (strong obligation)
We had to renew our medical licence before returning to the line. (strong obligation)