Can: present ability

Can describes a skill or ability that exists now. It is a modal verb: it has the same form for every person, it is followed by the base verb without to, and it makes questions and negatives without do.

subject + can / can't + base verb

• Our new manager can motivate people very well.
• I can understand Italian, but I can't speak it.
• Can your assistant use this software?

Could: general ability in the past

Could is the past of can. It describes what someone was generally able to do over a period in the past — a skill they had whenever they needed it — often with phrases such as when I was young, at school or in those days.

subject + could / couldn't + base verb

• When I was a student, I could remember whole pages of text.
• In her youth, she could dance all night.
• He couldn't swim until he was twelve.

With see, hear, smell, feel, understand and remember, could is also used for one particular moment: I could hear voices in the corridor.