"So" = extremely: I feel *so* tired, I could sleep for a week.
"Too" = excessively: I am *too* tired to even think about sleeping.
"Too" = excessively: I am *too* tired to even think about sleeping.
After CAN, COULD, WILL, WOULD,SHALL,SHOULD, MAY, MIGHT (and their negatives) use the base form of the verb:
➖ He CAN *swim* (not “swims”).
➖ He COULD *swim* (not “swims or swam”).
➖ He CAN *swim* (not “swims”).
➖ He COULD *swim* (not “swims or swam”).