Ago
Expression of time + ago
1) An expression + ago refers to a finished time, and is normally used with a past tense, not a present perfect.
- I met her six weeks ago.
- I met her a long time ago.
- She phoned a few minutes ago.
- He was watching the television ten minutes ago.
Last
1) Last is used without the article "the". Last week, last month etc is the week, month etc just before the one in which the words are said or written. These time expression are normally used with past tenses and without articles and without prepositions.
- I had a cold last week.
- Were you at the meeting last week?
- We bought this car last year.