Per Wikipedia, can is a "defective verb"...
For example, can lacks an infinitive, future tense, participle, imperative, and gerund. The missing parts of speech are instead supplied by using the appropriate forms of to be plus able to. So, while I could write and I was able to write have the same meaning, I could has two meanings depending on use, which are I was able to or I would be able to. One cannot say I will can, which is instead expressed as I will be able to.