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