Subject-verb agreement with other verbs

• When you use a verb in the present simple and the subject is the third person singular (he, she or it), add an –s to the verb: 

Professor Jones lives nearby. 

• Remember that there can only be one –s ending: either on the subject or on the verb: 

That car costs too much. 

Those cars cost too much. 

• Remember also that you must not add an –s to modal verbs (can, could, will, would, shall, should, must): 

It can be rather difficult.