• We use ago to talk about a time in the past.

We use it with the Past Simple: I went to Mexico two years ago.

(= two years before now).

• We use last to say the day, week, etc. in the past that is nearest to now: I went dancing last Saturday.

(= the Saturday before now).

• We use last with days (last Tuesday), months (last March) and in these phrases: last night, last week, last weekend, last month, last year, last century.

• We say last night, but we say yesterday morning/afternoon/evening NOT last morning, etc.

• We don’t use a preposition with last: last weekend NOT in last weekend, last month NOT at last month.

• We use in with years (in 2005) and months (in May).

• We use in the with decades (in the nineties) and centuries (in the twenty-first century).

• We can use on with days (on Monday) to mean last: I met him on Monday. = I met him last Monday.

• We use at with times (at ten o’clock).