Memory Game Modals of obligation/necessity (D)Online version Let's work on some modals! (should, can, need, have to, must) by Tami Sotelo Necessity. Strong. Optional, absolutely no obligation or phohibition of any type. Obligation, not as strong. Suggestion. Not an obligation, but it is best to do as adviced. Suggestion. Not a prohibition, but it is better not to do it. Prohibition, strong. Obligation, strong. Optional, not necessary but not wrong to do or have.