CHAPTER 10.  MODAL VERBS
There are nine modal verbs in English: can, could, may, might, must, shall, should, will, and would. Two of these, will and would, have already been discussed in detail.
 
1. Formation of the modal conjugations

All of the modal verbs are used as auxiliaries, and all of them form conjugations in the same way. Thus, the other modal auxiliaries form conjugations in the same way as will and would. For instance, the conjugation of the modal auxiliary could with the verb to work is formed as follows:
Conjugations of the modal auxiliary Could with the verb To Work
Simple Continuous
  I could work   I could be working
  you could work   you could be working (Devamini Okuyun…)