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