Memory Modals of obligation/necessity (D)Version en ligne Let's work on some modals! (should, can, need, have to, must) par Tami Sotelo Optional, not necessary but not wrong to do or have. Necessity. Strong. Optional, absolutely no obligation or phohibition of any type. Obligation, 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. Obligation, not as strong. Prohibition, strong.