No cambian nunca. Sin -s en tercera persona, sin -ing, sin pasado con -ed.
he cans · she musts · he can · she must
Detrás va el verbo tal cual, sin to, sin -s, sin -ing.
I must to go · she can plays · I must go · she can play
No usan do. Niegan con not y preguntan poniéndose delante del sujeto.
Do you can swim? · I don't can · Can you swim? · I can't
Se parecen a los modales, pero casi todos llevan to, y have to sí se conjuga (has to, had to, don't have to). Needn't es un modal normal.