Rules

1) The auxiliary verb is immediately followed by the main verb in its base form (without to)

Example: Maxwel can play piano. ✅

                  The boy can to play piano ❌

Note: We say  Can play, not can to play

2) Modal verbs do not change their form when used with the third person singular subjects (he/she/it). Therefore “-s” is not added to the modal verb

Example: This bird could fly when its wings weren’t injured✅

                  The bird coulds fly when its wings weren’t injured ❌

Remember: We say he could, she could, he could not he coulds, she coulds, it coulds.