Can't or Couldn't - What's the Difference? Basic English Grammar

In this video, Alisha answers 4 questions. - What's the difference between Task, duty, job, and assignment? How do we use them? - What's the difference between just and only? - How does the letter ""T"" sound in the words: ""party"" and ""started"". - What idiom can I use to describe past work if its results were unuseful and unnesessary and there was no practical outcome, but it wasn't clear from the start?