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. Optional, absolutely no obligation or phohibition of any type. Optional, not necessary but not wrong to do or have. Obligation, strong. Necessity. Strong. Prohibition, strong. Suggestion. Not a prohibition, but it is better not to do it. Obligation, not as strong.